CHAPTER VII.

THE WONDERFUL DIFFUSION OF HEAT.

When Humphry had written thus far concerning the sources of heat (for the boy was delighted to note down his thoughts on the various subjects he was studying),[32] he began to ponder over the several modes in which heat was _communicated_ to bodies removed from the different sources of it; for, said he to himself, “if there had been no means of propagating heat from one part of space to another, the fires could not have warmed us, and the sun would have been only a moon to our globe, while we should have been deprived of some of our most agreeable sensations. Hence, in the consideration of such a subject, it becomes necessary to attend to, and distinguish between, the several ways in which a body at an elevated temperature communicates its heat to others that are either in contact with it or at a distance from it, as well as the several conditions which determine the reception or absorption of the heat by different substances.

“Now, heat may be communicated from one body to another in three different ways—

1. By _emission_ of rays of heat from a distance;

2. By _conduction_ along the particles of a solid body;

3. By _convection_ or circulation among the particles of a fluid.

“The propagation of heat by the emission of heat-rays from a warm or burning body at a distance, is the one that first demands attention. This mode of communication is generally styled ‘_radiation of heat_,’ and it is evident that the heat-rays emanating from one body may be communicated to another, either directly, by the process of _transmission_ through the intervening substances, or indirectly, by _reflexion_ from the surfaces of those opposing them; for the heat-rays, like those of light, always proceed in a straight line, and are susceptible, likewise, of being reflected or driven off at an equal angle from polished surfaces.”

Having settled thus much in his own mind, and arranged the subject with that logical precision which was a marked feature in the genius of the youth, he proceeded to test experimentally the emissive energies, or, in other words, the radiating powers of different substances.

For this purpose he provided himself with a square tin canister: one of the four sides of this he brought to as high a polish as he possibly could; the second side he coated with a mixture of lamp-black and gum-water; over the third side he pasted a piece of paper, and the fourth he covered with glass. Then, having provided himself with a thermometer from the surgery below, he proceeded to arrange the canister at some distance from the thermometer, but on a level with it. After this he filled the canister with boiling-hot water, and then proceeded to note how the thermometer was affected when the canister was turned round, and each of its sides successively brought before the instrument. The boy soon ascertained, to his great joy, that the heat was thrown off most rapidly from the blackened side of the canister; next to that, he found the surface covered with paper to radiate heat more rapidly than the other two; then the glass side was discovered to possess more emissive power than the polished surface, while the polished surface itself had the least radiating power of all.

Delighted with the result of the experiment, and pleased with the knowledge it gave him as to the emissive energies of different substances for heat, the boy, to assure himself that he was not mistaken, held his hand at a short distance from the canister, and caused the differently-coated sides to pass successively before it. As he did so, he could feel the heat increase gradually as the polished side passed from before his hand and the blackened one came round in front of it; so that, had he not been aware of the fact, he would hardly have believed that the water in the canister was as hot at that part where the bright tin had been left as it was where the side had been blackened over.

Next the lad tried another modification of the same experiment. Having blackened one canister entirely over, and brightly polished the outside of another which was of the same size, he filled the two vessels with boiling water, and putting a thermometer into each he placed them upon a table at opposite corners of an empty room, and then found that the thermometer in the blackened vessel fell much quicker than that in the polished tin one; so he now saw that the reason why the _black_ side of the canister in the first experiment felt hotter than the _polished_ surface did to his hand was, that the water there was parting with its heat at a more rapid rate, so that in the entirely blackened vessel it necessarily cooled down sooner than in the bright tin one.

Humphry was so delighted with the truths he had thus discovered, that he tried a number of other experiments as to the radiating power of different substances, and at last came to the conclusion that lamp-black, sealing-wax, wool, paper, glass, and black-lead, were much better radiators of heat than the metals; their power of giving off heat being in the order in which they are here mentioned—a surface of lamp-black cooling quicker than one of sealing-wax, and sealing-wax again more rapidly than writing-paper, and so on down to the metals, which cooled the slowest of all.

Then, having dealt with _different_ substances, the youth set to work to ascertain what effect an alteration in the arrangement of the surface produced in the radiating power of the _same_ substance. Accordingly he tarnished, by means of acid, one of the sides of the polished tin canister he had previously employed, and found, on filling the vessel again with hot water, that the _dulled_ surface parted with its heat quicker than the _bright_ one. After this, he proceeded to roughen another of the sides with some emery paper, and then, on re-filling the vessel, he discovered that the scratched surface cooled at a greater rate than the smooth polished one.

“So, then,” said young Humphry to himself, “not only have _different_ substances various radiating powers for heat, but also a difference in the arrangement of the surface of the _same_ substance is attended with a like effect.”

Still the lad had to examine the result produced by bodies of _different densities_, and this he did by means of a vessel of cast-iron and one of wrought-iron, when he found that the _cast_ metal parted with its heat quicker than the hammered or _wrought_ metal; so that it was evident a _lighter_ material was a better radiator of caloric than a _heavier_ one—for the particles of the iron in being wrought had been brought closer together, and the metal thus rendered of greater density.

This done, Humphry made an entry in his note-book, “that not only did rough or dull surfaces part with their heat quicker than smooth or bright surfaces, but that light bodies were better radiators than heavy ones.”

The young experimentalist was overjoyed with the progress he had made, and he would have rambled off into a number of speculations as to the effect which the principles he had discovered must produce in nature (for he saw that the different surfaces of different countries must yield a like result); but he was too intent on pursuing the investigations he had undertaken to allow himself, yet awhile, to apply them to the explanation of terrestrial phenomena. Moreover, he had still to learn the different rates of cooling among bodies in the air and in a vacuum. To do this, however, an air-pump was necessary, and how he was to obtain such an apparatus puzzled his ingenuity for a considerable time.

At length the youth remembered to have seen a large syringe among Mr. Tonkin’s instruments, and having obtained the loan of this, he applied it to a stand, and used it as the pump for extracting the air from the receiver. When the instrument was complete, Humphry found that bodies which took between two and three minutes to cool in the air were as long as five minutes in parting with the same quantity of heat in a vessel from which all the air had been exhausted. So he now perceived, that the same substances gave off their heat twice as quick in the _open air_ as they did _in vacuo_.

The next step was to ascertain the different amounts of radiation among different bodies on the earth. For this purpose the boy borrowed as many thermometers as he could procure among his friends in the town; and early one evening, after the sun had declined, and when the soil was parting with the heat it had received in the course of the day, he proceeded to test, by means of the instruments, the several rates at which the various substances upon the earth were being cooled down. One of the thermometers he suspended in the air four feet above the grass-plat in the garden at the back of the doctor’s house; another he placed on some wool which he had spread on a raised board; another he deposited on the surface of the raised board itself; and a fourth he rested on the grass-plat. Shortly afterwards he proceeded to note the temperatures indicated by the several thermometers in the different situations, when he found that the one in the air stood at rather more than 60°, while that resting on the wool was at 54½°, and that lying on the board at 57°, whereas the one on the grass-plat marked only 51°. In an hour or two after this, the boy noticed that the blades of grass were suffused with dew, and that the fibres of the wool also were beaded over with little drops of moisture, but to a less extent than the grass, while the surface of the board remained almost dry.

“So then,” he said to himself, “_wool_ is a better radiator than _wood_, and, cooling quicker, condenses the moisture of the air more rapidly upon it; but _grass_ again, as the thermometer showed, cooled quicker even than the wool, and therefore collects more dew than either.”

This induced young Humphry to try another experiment, in order to ascertain whether those bodies which cooled most rapidly collected the most dew on their surfaces. Accordingly he placed a piece of bright polished metal and a piece of glass (for the surfaces of these substances were nearly the same) on the gravel-path, and was delighted to perceive that in a short time the _glass_ was covered with moisture while the _metal_ remained perfectly dry. A strip of _flannel_ was then put beside the other two, and, being a good radiator, it soon became spotted with dew-drops. After this the boy coated the piece of polished metal with lamp-black, and found it _then_, like the others, capable of condensing the moisture of the air upon its surface.

“It is as I expected,” cried the lad; “the dew which the ancients imagined to be shed from the stars is simply the condensation of the vapour in the atmosphere upon cold surfaces; and, consequently, those bodies which have the greatest radiating power, and so become cold the quickest, are found to have the largest deposition of dew formed upon them, while those which, like polished metals, part with their heat but slowly, and so remain for a long time at the same temperature, have but little moisture condensed upon their surface. The deposition of dew,” he went on musing, “is precisely similar to the condensation of moisture that occurs on the outside of a bottle of very cold water when brought into a warm room. The cold surface of the glass abstracts heat from the vapour in the air of the apartment, and so causes it to be condensed in the form of little watery globules on the surface. In the same manner the earth, parting at night with the heat it has received during the day from the sun, becomes cooler than the atmosphere above it—for thermometers show, that when the grass is at 51° in the evening, the air only four feet above it is more than 60°—and accordingly the cold surface of the blades acts upon the vapour in the atmosphere, precisely the same as the outside of the cold bottle does upon the air in a warm room.”

So pleased was the lad with the insight that his investigations had given him into some of the mysteries of nature, that he continued his experiments on this subject for many nights; and in the course of these he found, that not only had different bodies different dew-collecting powers, but that different colours even possessed the same property; for on exposing a piece of yellow, of green, of red, and of black glass to the night air, he perceived that the moisture appeared first on the _yellow_ glass, then on the _green_, but that none at all showed itself on either the _red_ or _black_ glasses.

To his astonishment, however, he at length discovered, that when the evenings were _cloudy_, and there seemed to be a greater quantity of moisture in the atmosphere, the pieces of flannel and glass, and little piles of swan’s-down with which he had studded the gravel-walk, remained unmoistened with dew; whereas, when the nights were _clear_ and apparently _dry_, they, one and all, with the exception of the polished metals, became rapidly suffused with moisture. This for awhile entirely baffled the boy’s comprehension. “How came it that more dew was deposited on dry clear nights than on dull damp ones?” Surely, such being the case, the dew cannot be said to proceed from the vapour in the atmosphere; for if it does, reasoned Humphry, it is evident that when there is more moisture in the air there should be more dew deposited on the earth.

At length it struck the boy that, perhaps, the clouds themselves might interfere in some way or other with the result; so the next fine clear night he strewed the gravel-walk, as before, with fragments of such substances as he had already found to be the best collectors of dew; and then, at the other end of the path, he placed pieces of the same substances under a small awning, which he made out of his pocket-handkerchief, fastened at each corner to a short stick. This he did in order to see what effect would be produced by _screening_ bodies from the sky—since the clouds, he fancied, might act in some such manner.

On returning to the garden after a short interval, Humphry was rejoiced to find that there was a copious deposition of dew on the pieces of glass and wool that he had left _exposed_ to the sky, while the surfaces of those which were _screened_ by the little awning above them remained perfectly dry.

“Yes,” cried the lad, “the clouds _do_ act as screens. They give back, perhaps, some of the heat that the earth at night is radiating into space, and so prevent bodies cooling down as rapidly as they otherwise would.”

However, to satisfy himself that the clouds really _did_ interfere with the radiation of bodies on the earth, Humphry arranged an apparatus for testing the point. This consisted of a thermometer, the bulb of which was first incased in wool (for that substance he knew to part readily with heat) and afterwards fixed in the focus of a small concave mirror. Then on the next windy night, when the clouds were drifting swiftly across the sky—leaving the heavens occasionally clear, and occasionally hiding the light of the stars—the anxious lad turned the mirror towards the blue vault above, and, on doing so, he could hardly repress his glee as he beheld the quicksilver in the tube of the thermometer descend and ascend, each time the sky became clear or clouded. Though, by means of another thermometer, he knew the temperature of the surrounding atmosphere to be 60°, Humphry nevertheless found that, when the sky was _unclouded_, the mercury in the one attached to the mirror indicated only 45°, whilst immediately that a _cloud passed over the firmament_, and so prevented the bulb from parting with its heat, the quicksilver rose rapidly again to the temperature of the air around. So intensely did Humphry exult in the result of this experiment, that he remained long, watching the thermometer rise and fall, as the clouds swept one after another across the sky.

The next day, Humphry, now that he had made himself acquainted with the circumstances that regulated the _emission_ or radiation of heat from bodies, began to turn his attention to the _reflexion_ of it from such substances as impeded the progress of the rays; “for,” said he, “if bodies at an elevated temperature have the power of sending out rays of heat in all directions—in the same manner as luminous bodies emit rays of light—it follows that substances opposing the passage of the heat-rays must either _absorb_ them, and so become heated themselves—or they must _transmit_ them and so allow the rays to proceed in their original direction—or else they must _reflect_ them and so bend them into another course.”

For the study of the _reflexion_ of heat the lad procured two concave mirrors, made of tin-plate and about 1 foot in diameter. These he arranged so as to slide up and down a pillar, to which they were respectively attached. Thus provided, Humphry proceeded to place a small “Florence flask,” filled with hot water, in the focus of one of the mirrors, while in the other focus he arranged a thermometer after this fashion:

[Illustration]

Now, though the mirrors were some feet apart, the mercury in the tube, to the boy’s great delight, rose almost to the heat of the boiling water in the flask.

After a few moments’ reflection, the lad fancied the effect might perhaps be due to the radiation of the heat from the flask itself, rather than to the reflexion of it from the mirrors. So, to satisfy himself whether or not such were the case, he placed a sheet of pasteboard immediately in front of the mirror near the thermometer, and thus prevented any rays being reflected from the one to the other. No sooner, however, had he done so than the mercury was seen to fall in the tube—even though the source of heat was as near to the thermometer as before; but directly he removed the pasteboard from between the mirror and the thermometer, the quicksilver rose rapidly again, and stood at the same number of degrees as it previously did.

Having convinced himself upon this point, he then drew the thermometer away from the focus and nearer to the heated flask, so that, if the effect were due to radiation, the mercury, as it approached the source of heat, should rise higher in the tube. The contrary result, however, was found to ensue; and it will be seen on reference to the preceding engraving, that by _radiation_ only a few of the heat-rays (which are indicated by the diverging _unbroken_ lines) would fall upon the thermometer, whereas by _reflexion_ a much larger number of such rays become concentrated upon the bulb in the focus of the opposite mirror—as shown by the _dotted_ lines in the diagram.

[Illustration]

Humphry was now anxious to see whether, by reflexion of the heat-rays, he could ignite combustible bodies at a distance; but for this purpose he changed the situation of the mirrors, arranging them vertically one above the other, instead of horizontally, or each on a level with the other, as before. Then he made a small basket of iron wire, and having filled this with burning charcoal, he suspended it below the upper mirror, so that it hung exactly in its focus, whilst above the lower mirror he fixed a small piece of phosphorus, and this was exactly in the focus also. Thus, on the completion of the arrangement, the boy was as astonished as he was delighted to perceive that the phosphorus was immediately inflamed by the _reflected_ rays of heat. Some fulminating silver was then exploded in like manner. After this Humphry boiled some water in a flask that he substituted for the piece of phosphorus in the focus of the lower mirror, and finally cooked a chop, by the same means, at some considerable distance from the fire.

Next, instead of the two mirrors, he rolled up a sheet of bright gilt paper, with the metallic side inwards, into the form of a long cone or funnel, so that the opening was larger at one end than at the other; then holding the larger end towards a clear fire, he found the rays of heat were concentrated into a focus at a little distance beyond the smaller end, and there he caused a bit of phosphorus again to inflame, by means of the reflected heat. The subjoined diagram exhibits the arrangement.[33]

[Illustration]

Humphry now began to wonder what effect would be produced by a piece of ice placed in the focus of one of the mirrors; and he thought for a long time whether the rays of _cold_ would be reflected from the ice, as those of _heat_ had been from the hot water and the burning charcoal. As the winter had long set in, he found no difficulty in obtaining such a piece as he required from one of the neighbouring ponds, and then arranging the mirrors as before, he placed it in the focus of one of them, while in that of the other he fixed the thermometer which he had previously employed.

To the lad’s astonishment he discovered that the mercury immediately began to fall, and at length stood at 32°, or the freezing point. “So then!” he cried, “it _is_ possible to reflect rays of _cold_ as well as those of _heat_. And yet,” said he to himself, after musing for a while, “_is_ it the ice, after all, that is radiating _cold_ to the thermometer, or the thermometer itself, which, being warmer than the frozen water, is really and truly radiating _heat_ to the ice?” If, instead of the thermometer, he had placed a red-hot body in the one focus, while the ice remained in the other, Humphry knew well enough that the warmer body, as it became cool, would be giving off heat to the colder one. “Why then,” he asked himself, “should he fancy that the thermometer itself—because it was _only a few degrees warmer than the ice_—lacked the power of parting with its heat to the colder body, in the same manner as the red-hot charcoal?”

The lad was soon convinced of his previous fallacy; and when he saw that the apparently contradictory effect was no anomaly after all, he could hardly refrain from smiling at the simplicity which had led him to believe at first that rays of cold were reflected from the ice to the thermometer, instead of the rays of heat being given off by the thermometer to the ice.

As yet, however, Humphry had experimented concerning the _reflexion_ of heat with mirrors only of _polished metal_; and one day, when he was recounting to Mr. Tonkin the curious effects he had produced, the old gentleman asked the lad what he imagined would have been the effect if, instead of _metal_ mirrors, he had used _glass_ ones.

Humphry answered confidently, that, as the results were due only to the reflexion of the rays from the concave surface, a glass mirror, _of course_, would have given precisely the same effects as the metal ones.

“Try it,” was all the old man said, as he smiled at the positiveness of the boy’s reply.

Nor was the young experimentalist long in doing so, for he saw by Mr. Tonkin’s manner that some strange difference in the effect would ensue—though for the life of him he could not divine what it was to be.

Accordingly, at the earliest opportunity, the boy substituted the glass concave mirror, which Mr. Tonkin had lent him for the purpose, for one of the metal ones which he had previously employed; then filling the little wire basket with red-hot charcoal, as before, and hanging it in the focus of the upper mirror, he once more suspended a piece of phosphorus in the focus of the lower mirror, which was now of _glass instead of metal_. To his utter amazement, however, the phosphorus was no longer capable of being inflamed in such a manner.

It was but the work of a moment to remove the combustible from the focus of the glass mirror, and to place a thermometer there in its stead; and this soon showed that there was now _little, if any_, heat reflected.

“How wonderful!” cried the startled boy. “What _can_ be the cause of it? I’ll arrange the mirrors differently,” he added, “and see if I can find it out.”

[Illustration: HUMPHRY’S EXPERIMENTS ON THE DIFFUSION OF HEAT.—Page 157.]

But no sooner did Humphry put his finger on the glass than he drew it suddenly back, as he exclaimed, “How hot the lower mirror has become! and I remember when I used the metal one, that I was surprised to find, on removing it, though the heat was sufficient to boil water and ignite bodies in its focus, the metal surface of the mirror itself was scarcely warmed. But now that _glass_ is used,” he went on, “_the mirror itself is rendered hot, while in the focus of it there is scarcely any perceptible increase of temperature_. So, then,” he added, “the glass _absorbs_ the heat-rays, and therefore does _not reflect_ them, while the metal on the other hand _reflects_, because it does _not absorb_ them. Still it’s very strange,” mused Humphry, as he proceeded to blacken a small card, “for the glass mirror must _reflect_ the _light_ of the fire, though it _absorbs_ the _heat_ from it. I’ll try whether such is the case or not.”

The card was then placed in the focus, and a bright spot of light was seen shining like silver in the centre of the blackened surface.

“Yes,” cried the lad, “it reflects the light, but not the heat of the fire. How strange! I wonder whether the same effect would be produced by the sun’s rays!”

Accordingly the next day, when the sun was shining brightly, Humphry arranged the mirror in the garden, so that the beams might be concentrated in its focus; and then, to his greater astonishment, he found that he could inflame combustibles by the _solar_ heat with the _glass_ mirror, in the same manner as he had previously done by _artificial_ heat with the _metal_ ones.

“The _light and heat_ of the sun, then,” said Humphry, as he stood watching the white fumes of the burning phosphorus rise in the air, “are capable of being reflected by glass, whereas the _light only_ of an artificial fire can be concentrated into a focus by it—the heat in the latter case being absorbed.”

The metal mirror, likewise, was found to possess the power of reflecting both the solar light and heat, in the same manner as it had been before made to reflect both the light and heat of an artificial fire.

The experiment with the glass mirror, however, clearly showed that solar heat differed in some way or other from terrestrial heat; but _how_, was a source of continual wonder to the lad.

* * * * *

From the _reflexion_ of heat, Humphry proceeded to the _transmission_ of it.

Light passes readily through certain substances, which are therefore said to be _transparent_, while others impede the progress of the beams, and are consequently called _opaque_. “Is there, then,” mused the boy, “such a property as _transparence_ and _opacity_ for _heat_, as well as light, among bodies? Are some substances _pellucid_, as it were, to heat like they are to light? and are some as impermeable to the one as they are to the other?”

The lad knew well that the heat of the sun was capable of being transmitted through glass as well as its light, for he had often concentrated the solar beams by means of a magnifying or “burning” lens, as it is called: glass, therefore, was transparent to the _solar_ heat as well as light; but was it so to the rays of _artificial_ heat?

To ascertain this, Humphry borrowed old Dr. Tonkin’s large reading lens, and held it before the fire so that the focus fell upon the bulb of a thermometer. But though the light of the burning coals was seen concentrated into a bright spot upon the bulb, still the mercury in the tube gave no indications of any increase of temperature. The lens, however, which was scarcely warmed when the sun’s rays passed through it, became greatly heated when the rays of the artificial fire were made to fall upon it—thus showing, that while it _transmitted_ the solar heat it _absorbed_ the terrestrial.

It was evident, therefore, that though the _heat of the sun_ has the power of passing freely through glass, _artificial heat_, on the other hand, is completely stopped by it.

Humphry then thought he should like to try the effect of a piece of black glass, for this would be perfectly opaque to light, and he longed much to see whether it would be equally impermeable to heat. On holding a square piece before the fire, the boy was surprised to perceive the thermometer he had arranged behind it rise rapidly, thus showing that though black glass was _in-transparent to light_, it was by no means _opaque to heat_. That the quicksilver was made to mount in the tube solely by the influence of the heat-rays which traversed the black glass—and not by any indirect radiation from the fire—Humphry assured himself, by placing a piece of white glass, of the same size and thickness as the black one, before the thermometer: the quicksilver, however, was immediately seen to fall. “How marvellous is this!” he exclaimed. “Light and heat, then, are capable of being separated one from the other; and there are bodies in nature which, like _white_ glass, are _transparent_ to light, but _opaque_ to heat; while there are others, like _black_ glass, that allow the heat-rays to _pass through_ them, though they are _incapable of being traversed_ by the luminous ones.”

The boy was so full of the new truth that had thus become impressed upon his mind, that he hurried off to Mr. Tonkin to confer with him on the result. From him Humphry learnt that there were other substances, besides glass, that gave equally curious effects—the most striking of these, the old gentleman told the boy, were _alum_ and _rock-salt_, for though both were transparent to light, they had by no means the same power of transmitting heat; for it would be found that while a small plate of rock-salt allowed the rays of heat to pass almost _freely_ through it, a similar plate of alum was nearly _impermeable_ to them.

The young philosopher was not long in trying the experiment. Having procured two such plates as Mr. Tonkin had advised, he used them as small screens in front of the fire, and found that a thermometer behind the rock-salt rose rapidly; whereas, behind the alum, it was scarcely affected, for the heat was nearly all stopped by it.

The possibility of separating heat from light made a powerful impression upon the ardent boy, and he wondered whether he could arrive at the same result with the solar beams as he had with the rays of an artificial fire. For a long time he pondered over the matter, and conceived and tried a number of fruitless experiments in connexion with it.

At length, however, he remembered to have read somewhere, that by means of a glass prism the beam of white light proceeding from the sun might be separated into all the colours of the rainbow.

Accordingly he set to work to repeat the experiment. Having darkened his room he made a hole in the window-shutter, and placed behind it a glass prism, with one of the sharp edges downwards and one of the flat sides uppermost, as shown in the annexed illustration:

[Illustration]

Immediately that the arrangement was complete, and the beam from without fell on the glass within, the wall on the opposite side was iridescent with a strip of variegated light, as if a slice of a bright rainbow were clinging to it. The lower end of the luminous band was a rich warm red, and this passed, by a tint of orange, into a bright yellow, which again died away, by deepening hues of green, into a narrow strip of dark blue, while, at the upper end, the indigo tint became warmed into a brilliant edging of violet.

When the rapture of the boy on first beholding the sight had, in a measure, subsided, he proceeded, by means of a thermometer, to ascertain the temperature of the several rays. First he tried the upper end of the spectrum, and found that in the _blue_ ray the mercury marked 56°. Then passing downwards Humphry was overjoyed to see the quicksilver mount as he proceeded towards the middle, where, in the _yellow_ ray, the instrument indicated a temperature of 62°, _i.e._ 6° higher than in the blue; while at the lower end—at the extremity of the _red_ ray—the temperature was found to be as high as 79°, _i.e._ 17° higher than it was in the yellow.

There was then, altogether, as much as 23° difference between the heat at the extreme ends of the luminous band—the _red_ ray being upwards of half as hot again as the _blue_ one—so that light and heat were capable of being separated even in the solar beams themselves; for the yellow contained the most light of all, and yet it was 17° colder than the extreme verge of the red ray, where there was only a faint luminous blush to be perceived.

* * * * *

The next step was to ascertain the circumstances regulating the _reception_ or _absorption_ of heat.

Humphry had now investigated the laws which governed the radiation or _emission_ of the rays of heat from bodies at an elevated temperature. He had ascertained that these rays not only emanated from heated substances at different rates, and so caused them to cool down more or less rapidly, but that—though their tendency was to proceed, like the rays of light, in a straight line—they were capable of being _reflected_ or bent back by certain bodies opposing their progress, and that in such cases the reflecting bodies themselves did not become heated by them. Other bodies, again, he had found to have the power of _transmitting_ the rays of heat, that is to say, of allowing them to pass _through_ their substance rather than reflecting or driving them _back_ from their surface; and such transmitting bodies, moreover, were likewise scarcely warmed by the heat that traversed them. Now he was about to investigate the conditions that determined the _absorption_ of the heat-rays, by bodies upon which they fell after being given off by _radiation_ from others of a higher temperature.

The lad’s first experiment upon this subject was to blacken the surface of one of the metal mirrors that he had previously found to reflect the heat, without being itself warmed in so doing.

The result proved to be as Humphry had anticipated. The mirror no longer had the power of concentrating the heat in a focus, at a short distance in front of it: for now, instead of _reflecting_ the rays and remaining _cool_ as before, it _absorbed_ all the heat that fell upon it, and became itself _warmed_ by the neighbouring radiator.

The same effect ensued when the surface of the mirror was whitened with chalk, and the same again when it was roughened, or scratched, with emery paper: so that _rough_ and _dull_ bodies proved to be better absorbers of heat—even as they were better radiators—than _bright_ or _polished_ ones.

Hence there appeared to be some connexion between the radiating and absorbing powers of different substances—those which cool the quickest seeming to be capable, also, of being heated in the shortest time.

To test this the lad placed a blackened and a bright-polished vessel in front of the fire, and found that the thermometer in the _black_ vessel rose much more rapidly than did that in the _bright_ one.

Humphry then availed himself of these two vessels as a means of testing the relation between the _absorbing_ and _radiating_ powers of black and bright-polished surfaces. Into the mouths of the black and the bright tin vessel he inserted a thermometer, and then placed between them one of the square canisters he had previously employed, and which, it will be remembered, had one of its sides bright, while the opposite one was coated with lamp-black.

Having filled the middle canister with boiling-hot water, he proceeded first to note the radiating and absorbing effects when the _different_ surfaces were opposed to each other. On arranging the middle canister so that its black side was turned towards the polished vessel at one end, and its polished side to the blackened vessel at the other end, there was no effect produced upon either of the thermometers; for then the opposite powers of the _different_ surfaces exactly balanced each other. When, however, the apparatus was so adjusted that _similar_ surfaces were opposed—that is to say, so that the blackened side of the canister in the middle was turned towards the black vessel, and the bright-polished side to the bright-polished vessel—the thermometer in the black vessel immediately indicated a great excess of heat; for then not only was there a _good radiator_ opposed to a _good absorber_, but, on the other side, the two bright surfaces were facing each other—that is, the _bad radiator_ was turned towards the _bad absorber_—so that even the little heat which was given off from the polished side of the canister was driven back again to it by the surface of the neighbouring bright vessel. Hence everything _favoured_ the radiation and absorption of the heat on the one side, where black was opposed to black, and _prevented_ it on the other, where metal was facing metal; and thus the great elevation of the thermometer was accounted for.

As it was now winter time and the snow lay thick upon the earth, Humphry availed himself of the circumstance to test the absorbing powers of different _colours_. For this purpose he took a number of pieces of different coloured cloths, and placing them at mid-day upon the snow, so that the sun’s rays could fall directly upon them, he found that the _dark_ colours sank the deepest into the frozen mass beneath, while the _lighter_ hues produced scarcely any thawing effect, and the _white_ remained utterly inactive.

The same result was obtained by means of coloured glasses; for against a window-pane that was covered with hoar-frost the lad placed some pieces of black, red, green, and yellow glass, and the consequence was, that the ice opposite to the _black_ and _red_ pieces was melted long before any thawing effect was visible upon the frozen film screened by the other colours.

When the weather grew warm Humphry obtained another very curious illustration of the power of black substances to absorb the heat of the sun’s rays. Having filled a glass tube with spirits of wine, he placed it in the focus of a lens, and found that the solar heat traversed the transparent liquid without warming it. On immersing a small piece of charcoal, however, in the alcohol, so great was the absorptive power of the _black_ surface that the fluid immediately began to boil. By the same means, too, he succeeded in raising the temperature of water to the boiling point. This showed that water, as well as spirits of wine, was a good _transmitter_ and bad _absorber_ of heat; that is to say, that the rays passed freely through each without warming either, unless some substance were immersed in the liquid in order to detain and absorb their heat.

_Air_, on the other hand, the boy knew to have little or no heat-absorbing power; for the rays emitted by a distant hot body traversed the atmosphere without sensibly raising its temperature. He had read, too, that philosophers had calculated that only one-fifth of the solar heat was absorbed in passing through 1000 feet of the air, and that but one-third of the entire heat of the sun was taken up by the passage of the beams through the whole atmosphere.

Humphry, moreover, sought to discover whether the sun’s heat, reflected from a mirror, would produce the same effect as the direct solar beams. Accordingly, before the winter passed away, he placed two pieces of blackened card upon the snow, at a considerable distance apart. One of these he left exposed to the _direct_ rays of the sun, while upon the other he caused the sunbeams to fall _indirectly_, by reflecting them from a polished metal surface. The black card that was submitted to the direct solar beams sank, after a little time, deep into the snow, while the frozen mass around—though the beams fell full upon it—was but slightly thawed. With the black card, however, upon which the sun’s rays were _reflected_, a precisely opposite result ensued. In that case the surrounding snow itself was the first to melt, while the blackened surface seemed to have been deprived of its power of absorbing heat, and remained high on the unthawed pile beneath it.

Further, Humphry noticed that the snow which lay near the trunks of trees, or wooden posts, melted much sooner than that which was at a distance from them, and that the thawing always commenced at the side facing the sun. Hence it was evident that the solar heat, after being either _reflected_ or _radiated_ from bodies on the earth, and so made to fall _indirectly_ upon other bodies, was rendered capable of being absorbed by substances which, like snow, had but little or no power of being warmed by it _directly_.

Why this should be, or what alteration the solar rays underwent in impinging upon terrestrial bodies, so that substances which before absorbed the sun’s heat with difficulty became afterwards more easily warmed by it, was more than Humphry’s philosophy could explain—though it cost him many a day’s hard thinking in trying to account for the result.

* * * * *

Having now investigated the conditions which governed the diffusion of heat from a _distant_ point, Humphry next proceeded to inquire into the circumstances which regulated the communication of heat to bodies in _contact_ with others at an elevated temperature.

This constitutes what is called the _conduction_ of caloric, and occurs between different bodies, or parts of the same body, immediately adjoining each other. The communication of heat by _conduction_ is a slow process compared with that of _radiation_, which is, probably, as rapid as the diffusion of light itself. Philosophers have calculated, that even if the crust of the earth were made of cast-iron (which is a much better conductor than rocks and stones), it would take myriads of years to transfer the heat from a depth of 150 miles below the surface to the surface itself; whereas by radiation the solar heat travels from the sun to us in 8½ minutes.

The laws which regulate the communication of caloric to _distant_ objects are similar to those which would ensue if the heat really consisted of so many hot particles darted out from the heated body in all directions; and colder bodies placed in the neighbourhood of heated ones, either become hot in the same manner _as they would if_ such particles were positively absorbed by them, and entered into their substance; or they _reflect_ the heat to other bodies, while they themselves are unwarmed by it—_as if_ (according to the hypothesis) the caloric particles were elastic, and had the power of bounding off from smooth surfaces interfering with their progress; or else they _transmit_ the heat rays, _as if_ the imaginary particles of caloric were capable of freely traversing certain substances, and that, also, without sensibly raising the temperature of the permeated mass.

But all bodies, or parts of bodies, which are in immediate contact with some other at a higher temperature, become themselves warmed; _not_ by rays thrown out from the heated mass, but by _conducting_ or diffusing the heat from one point to another, and so disseminating it ultimately throughout their whole substance.

Humphry was thus particular in impressing upon his mind the precise difference between the _radiation_ and _conduction_ of caloric before entering upon the study of the latter process; for he knew that without clear and distinct views upon the subject it was impossible for him to arrive at any absolute knowledge.

[Illustration]

To illustrate the _gradual_ progress of heat by _conduction_, the lad took a square bar of iron, about 20 inches long, and he attached to the under side of it (by means of a little wax) 10 small wooden balls, so that they were about 2 inches apart from each other. Then he heated one end of the bar in the flame of a lamp, and found that the balls fell from under it _one after another_, as the heat found its way along the metal and melted the wax below. The arrangement of this simple and instructive experiment is here shown.

[Illustration]

The next step was to learn the different conducting powers of different substances. For this purpose Humphry had several small metal cones made, all of the same size; one of these was of copper, another of iron, a third of zinc, a fourth of tin, a fifth of lead, a sixth of marble, and a seventh of brick. Then having tipped each of them with a small piece of wax, he stood them, all a short distance apart from each other, on the metal plate at the top of the iron stove by which Mr. Borlase’s surgery was heated. The result was, that the wax at the top of the _copper_ cone was the first to melt. Some little time afterwards, that at the apex of the _iron_ began to liquefy; and soon after the iron, that upon the _zinc_ was rendered fluid; while, shortly following the zinc, the wax on the _tin_ commenced trickling down the sides. A short interval elapsed, and then the cerate at the top of the _lead_ became fluid. Again a lapse of a few moments occurred, after which the wax with which the _marble_ cone was tipped began to flow; and, last of all, that upon the piece of _brick_ was liquefied.

The different conducting powers for heat among the several substances employed were thus made evident. The metals were more capable than either marble or brick of diffusing the caloric from one part of them to another; while among the metallic substances themselves copper was proved to be a much better conductor than iron; iron, again, a little better conductor than zinc; and zinc, too, slightly better than tin. Lead, on the other hand, was the worst metallic conductor of all.

The limited means of the young experimentalist, however (for Humphry was obliged to seek Mr. Tonkin’s assistance for any particular apparatus he required), did not admit of his testing the conducting powers of either gold or silver. But had he done so, he would have found that the precious metals were much better conductors than any other—_gold_ being the best of all, and _silver_ only a little inferior to it. _Platinum_, however, was a striking exception, its heat-conducting power being only a little superior to that of iron.

[Illustration]

Humphry after this sought to discover what would be the effect if he placed a good conducting metal in connexion with a bad one. For this purpose he employed a short curved bar of copper; and having heated it, he set it across the top of a small leaden pillar to cool, thus: when, to his utter astonishment, a series of musical sounds were given forth as the copper cooled, the tones now rising and now falling like those of an Æolian harp.

By the same means as Humphry had employed for testing the conducting powers of the metals, he ascertained that _wood_ was a very bad conductor of heat, and that the _lightest_ woods were the worst. _Charcoal_, too, he found to have but little power of diffusing the heat from one part of it to another. This explained to the boy the reason why a piece of charcoal, red-hot at one end, may be held—at a short distance even from the heated part—without burning the fingers.

Humphry now set to work to raise to a considerable temperature several pieces of such substances as he had ascertained to be good and bad conductors, so that he might learn what effect they respectively produced upon the touch when highly heated. As he had anticipated, the _bad conductors_—such as the wood and brick—could be handled without pain, whereas the _good conductors_—like the metals—burnt the fingers immediately they were brought into contact with them.

Pursuing this result, the lad, eager to display his knowledge to the servant of the house, took the boiling kettle from the kitchen fire, and, to the amazement of the maid, allowed the sooty bottom of it to rest upon his palm; for the crust of charcoal with which (by long usage) the vessel had become coated underneath—being a non-conductor—prevented the heat of the boiling water within being communicated to the hand.[34]

On recounting to Mr. Borlase the experiments he had performed concerning the conduction of heat, Humphry was informed by that gentleman that it was painful to touch good conductors like the metals when they were heated about 120°. Air, however, he told the boy, might have its temperature raised even to 300°, without producing any sense of burning; adding, that some eminent sculptors had large ovens in connexion with their studios, for drying the moulds they employed in bronze castings; and though these places were often heated far above the boiling point of water, the workmen entered, and remained there for some minutes without much inconvenience; and even persons unused to such high temperatures might walk in and out of the ovens with impunity, though to such any attempt to remain occasioned a difficulty in breathing and a painful sensation about the eyes. It was found necessary, however, under such circumstances, to carefully avoid _the contact of any good conductor_; for if, while in the heated oven, a piece of metal were touched, it would inevitably burn—even the coins in the pocket were sufficient to produce intense pain. “A story is told,” he continued, “of a person who once, inadvertently, entered such a place with his spectacles on, and these, being mounted in silver, soon blistered the parts of his face with which they were in contact. On the other hand,” proceeded the surgeon, “it has been found that in high northern latitudes, where the cold is sometimes sufficiently intense to freeze mercury—though this requires the temperature to be 72° below that required to freeze water—yet even such excessive cold may be borne without uneasiness, provided the air be tranquil, and the persons well clothed in good non-conductors, such as wool and fur. If, however, _metallic substances_ be touched at this low temperature, a sensation like that of burning is experienced, and the part quickly becomes blistered. The reason of this,” the doctor concluded, “is, that the heat, being as it were free to move in all those substances which are, like the metals, good conductors of it, is readily communicated to us by such substances when at a higher temperature than ourselves, while our heat is as readily abstracted by them when they are colder than we are. Hence good conductors, like metals, always feel colder to the touch than bad conductors, like wood or fur—even though these latter bodies can be shown by the thermometer to be of the same temperature as the others.”

Humphry’s conversation with the doctor induced him to try another experiment, illustrative of the conducting power of wood and metal. He took a small rod of polished brass, about a foot in length, and stretching a strip of writing-paper tightly over it at one end, he tried to burn the paper in the flame of a lamp, but discovered that it was impossible even to scorch it; for the heat, as soon as applied, was _conducted away_ so rapidly along the metal, that it prevented the temperature of the paper being raised sufficiently to char it. On substituting, however, a smooth piece of wood for the brass rod, he found that the paper stretched over the end of it soon began to scorch in the flame, and that the wood itself shortly became ignited in consequence of its bad conducting power, which, opposing the diffusion of the heat along it, concentrated the effects upon the spot to which the flame was applied.

After this, the boy began to turn his attention to the conducting powers of _liquids_, rather than _solids_, with which he had previously dealt.

[Illustration]

That liquids are very _imperfect_ conductors of heat, Humphry made out in the following manner: He filled a tumbler with water, and in this he placed a piece of “fusible alloy,” which is a composition of metals melting at a temperature below boiling heat. Then a thin copper basin was made to float on the surface; and into this he put some pieces of red-hot charcoal; so that, after a time, the stratum of water at the top of the tumbler began to boil; but, even though the upper part of the liquid was at boiling-point, so slight was the power of the water to _conduct_ the heat from one part of it to another, that the stick of alloy, which reached within an inch of the top, remained wholly unmelted by it.

[Illustration]

The same effect was found to ensue with heated _oil_, though this the lad tried in a somewhat different manner. In a thin glass tube a small quantity of water was frozen by plunging it into a mixture of salt and snow. Then, upon the lump of _ice_ at the bottom a small quantity of _oil_ was poured; and, lastly, upon the oil some _spirits of wine_ was made to float. The tube was now held over the chimney of a lamp, and the spirit made to boil until the whole was evaporated, when, on plunging a thermometer into the oil, it was found to be but slightly heated, while the ice itself had undergone no change, but remained still solid at the lower part of the tube.

Next, in due order, came the conduction of heat by _gases_ and _vapours_; and of this Humphry obtained a remarkable illustration in a fact which he learnt of the engineer at the Wherry Mine, who told the lad that high-pressure steam did not burn, though its temperature was some hundred degrees above that of steam at a low pressure. The scalding effect of the vapour at a low pressure, the man informed the youth, arose from the small particles of hot water that were diffused throughout it, and which, indeed, rendered it visible in the air; whereas in high-pressure steam no such watery particles existed, and the vapour was consequently not only imperceptible to the sight, but, being a bad conductor of heat, it had no more power to burn than so much hot air.

Again, that _gases_, in a state of combustion, are bad conductors of heat, Humphry was aware, from having repeatedly passed his finger through the flame of a spirit-lamp without burning it, and yet the temperature of such a flame might be shown to be many hundred degrees beyond that of a piece of red-hot metal. _Air_, again, he knew to have little or no conducting power; and he had heard from Mr. Tonkin that in Russia and other cold countries double windows, with a stratum of air between them, were used to prevent the heat of the apartment being carried off. So, again, in furnaces, double walls with a stratum of confined air in the middle are employed to stop the _egress_ of heat: even as in ice-houses the same means are adopted to stay the _ingress_ of it.

* * * * *

The diffusion of heat by the process of _conduction_, however, generally occurs among _solid_ bodies, in which the particles are more or less firmly united; but _liquids_ and _gases_ (where the particles, owing to the want of cohesion among them, are free to move) mostly became warmed by a very different process; that is to say, the heat applied to them is spread from one part to another—not by being propagated, as in solids, from one fixed particle to that which is next to it—but by the _motion or circulation of the heated particles themselves_, so that each in its turn receives a portion of the heat applied, and then giving place to another particle, the whole mass ultimately becomes raised to one uniform temperature by the _direct_ agency of the radiant body, rather than by the _indirect_ process of transference from atom to atom along the entire substance. The one process is termed the _conduction_ of heat, the other the _convection_ of it; and while the former prevails among the _cohering_ particles of solid bodies, the latter generally obtains among fluids whose atoms are _free to move_.

[Illustration]

In order to render visible this same circulation of the particles of fluids while in a heated state, Humphry bruised in a mortar a small piece of amber, and then having filled a glass tube with water, he threw in a few pinches of the powder, which, being nearly of the same specific gravity as the liquid, neither sank nor floated in it. Then applying a gentle heat to the centre of the bottom of the tube, the boy saw, by means of the amber-dust suspended in the fluid, that currents immediately began to _ascend_ in the middle of the water, and to _descend_ in it at the sides of the vessel—in the direction of the darts in the above engraving.

[Illustration]

If, however, he heated the sides of the tube, the currents were found to take a contrary direction, going _upwards_ at the sides and _downwards_ in the centre.

On continuing the heat, Humphry perceived the currents to become more and more rapid, till the water boiled, and when the whole of the liquid had acquired an uniform temperature, he observed that they ceased altogether. He then endeavoured to ascertain if it were possible to produce these currents in a liquid by heating it at the top, but the boy discovered, on applying a spirit-lamp to the upper part of the tube, thus—that though the top of the water was made to boil, and the amber-dust there thrown into rapid circulation, the particles at the bottom remained unmoved, the fluid below being undisturbed and cold.

[Illustration]

The reason of this was almost self-evident. The warm water was _lighter_ than the cold, and therefore _rose_ to the top immediately it became heated, while the cooler and _heavier_ portions _descended_ to occupy its place. Hence, in heating the tube at the bottom the current was observed to go upwards in the middle and downwards at the sides, these being kept comparatively cool by the action of the external air.

[Illustration]

Pursuing this subject, Humphry took a large and a small Florence flask, and into the mouth of the large one he fitted two long bent glass tubes, by means of a perforated cork and cement. These, together with the large flask, were filled with water and then made to dip into the open mouth of the smaller flask, which was likewise filled with water, but tinged a deep blue with indigo. One of the tubes was arranged so as to dip only about half an inch below the surface of the blue liquid, while the other descended nearly to the bottom of it, and was slightly curved upwards at its extremity. The arrangement will be readily understood by reference to the annexed engraving. On applying the flame of a spirit-lamp to the lower flask, the blue liquid was seen to ascend by the tube on the left side; then reaching the large flask at the top, it there circulated through it, in the direction of the darts, and descended by the other tube, back again to the small flask at the bottom. Thus a perfect circulation was seen to be kept up, and the heat, by means of _convection_, carried from one flask to the other.

[Illustration]

After this Humphry sought for some means of rendering the currents of heated air visible in the same manner. For such purpose he took a large glass jar, having a wide opening at the bottom and a narrow one at the top. Into the upper aperture he inserted a long lamp-glass, and down this he placed a diaphragm of card, so as to divide the glass chimney into two channels. Then the lad procured a shallow pan, and having poured a little water into it, he set a piece of lighted candle in it and covered it over with the jar and chimney, so that when the whole was duly arranged it appeared as here shown. Then having lighted a piece of brown paper and blown it out again, he held the smouldering end over the chimney, and saw, by the curling of the smoke from the paper, that the heated air from within was _ascending_ the lamp-glass by one side of the diaphragm, and _descending_ by the other, in the direction of the arrows in the illustration; whereas, when the card-board partition was removed from the chimney, the currents _ceased_, and the light was soon extinguished.

The boy applied the same simple means, likewise, to learn the direction of the currents of air on opening the door of a heated apartment, and found, by the smoke from a piece of smouldering paper, that at the _upper_ part of the door the heated air from within was rushing _outwards_, and at the _lower_ part the cold air from without was setting _inwards_, whilst at the middle scarcely any draught, one way or the other, was perceptible.

This naturally turned the boy’s attention to the subject of the wind, which appeared to him to be merely a vast current set up in the atmosphere by the heating power of the sun’s rays. He had noticed, too, that, shortly after sunrise, a breeze frequently sprang up at sea and blew towards the land, increasing as the day advanced, and declining and ultimately expiring at about sunset; whilst in the evening, after sundown, a wind often arose in the opposite direction—namely, _from_ the land _towards_ the sea—and lasted the whole of the night, ceasing only with the reappearance of the sun.

Humphry was therefore anxious to discover some experimental means of reproducing these effects on a small scale.

Having procured a large shallow milk-pan, he filled it with cold water, and then took a metal “hot-water plate,” and having poured some boiling water into this, he set it in the middle of the pan, saying to himself as he did so, “the cold water there, in the outer vessel, represents the ocean, while the heated metal plate in the centre stands for an island warmed by the rays of the sun; for the land, being a better absorber of heat than the sea, will have its temperature raised some degrees higher than the water in the course of the day.”

This done, the eager boy proceeded, by means of the smoke from a piece of smouldering paper as before, to discover the direction of the currents that would be set up in the air under such circumstances. As he held the smoking paper at the edge of the pan, Humphry was delighted to see the white fumes drawn towards the hot plate in the middle, or, in other words, _from_ the miniature ocean _towards_ the mimic island encompassed by it; and this he knew was precisely the current that was found to prevail throughout the day in tropical countries.

[Illustration: DURING DAY.]

Then, to impress the phenomena firmly upon his mind, the boy drew in his note-book the annexed diagrams, illustrative of the currents produced in the atmosphere by the _heating_ of the earth during the _day_, and the _cooling_ of it during the _night_. “But if the inequality of the temperature between the land and the sea gives rise to such results, how much greater,” mused the boy to himself, “must be the effect produced by the difference between the heat of the earth at the equator—where the average temperature is said to be 80°—and at the poles, where it is calculated to be as low as 56° below zero, the difference being as much as 136°! What a vast aërial current must be set up by such means!”

[Illustration: DURING NIGHT.]

Then the lad made another drawing, illustrative of the effect that would ensue under such conditions, and he set above it a series of arrows to show the direction of the currents that would be thus induced in the atmosphere. For the air, being heated by the vertical sun at the tropics, rises there, as it does up a chimney, while the colder air from the northern and southern hemispheres glides in from below, on both sides of the equator, to supply the place of that which has been made to ascend by the heat; precisely in the same manner as, when the fire burns, fresh air is continually rushing in under the door and windows. Then the heated air, after rising to a considerable height above the earth, at length flows over, as it were, and forms in the atmosphere an upper current _from_ the equator _to_ the poles, where it becomes cooled, and is then drawn down to supply the place of that which has been drafted _from_ the colder _to_ the warmer regions. “But,” said the boy, as he surveyed the drawing, “according to this the winds which are found to prevail in the tropics should blow _north_ and _south_; whereas they are found to come from the _north-east_ and _south-east_ quarters.”

[Illustration]

Humphry puzzled himself for a long time in endeavouring to explain the phenomenon, but it was more than his philosophy could accomplish; so he had to consult his old friend Mr. Tonkin again, and from him he learnt that the change in the direction of the currents is due to the motion of the globe and the unequal rates at which different parts of the earth’s surface revolve. Consequently, as the currents of air which set in towards the equator from the poles come from parts that revolve about the axis at a much _slower_ rate than the equator itself, they _hang back_, or _drag_, upon the surface, in a contrary direction to the rotation of the earth itself; so that, while the globe turns eastward, they acquire somewhat of a westerly course, and, appearing to come from the opposite quarter, assume, therefore, the character of permanent _north-easterly_ and _south-easterly_ winds.

But to make the matter clearer, Mr. Tonkin exhibited the following illustration to the boy, in which the effect of the earth’s motion in changing the direction of the atmospheric currents is immediately apparent.

[Illustration]

The old gentleman, however, informed Humphry that there are other terrestrial currents produced by the process of _convection_. In the ocean the same circulation of hot and cold streams is found to obtain; for the sea, warmed by the heated shores of the tropical regions, is made by _convection_ to move from the equator like a vast river, while from the poles an immense current of colder liquid streams forward to supply its place. For the same reason as was before explained in connexion with the trade-winds, the polar current, having a slower rate of rotatory motion, assumes, on reaching the equator, a westerly direction, and so flows in one broad stream across the globe; then, striking against the vast continent of America, it divides into two large streams. One of these flows southward down the eastern coast of Southern America, and finally enters the Pacific Ocean through the Straits of Magellan. The other turns northward, enters the Gulf of Mexico, sweeps round the coast in a powerful current known as the Gulf Stream, and then proceeds along the Northern American shores to the coast of Newfoundland, where it crosses the world again, and occasionally extends even to the western shores of the British Isles.

The direction of these oceanic currents is indicated in the subjoined chart:

[Illustration]

“There is, however,” continued Mr. Tonkin, “another great heat-stream traversing the earth, though this takes place within the crust itself, and is due more to _conduction_ than _convection_, as in the other cases. For philosophers tell us, that the daily impressions of heat which the earth receives from the sun, follow each other into the interior of the mass, like the waves which start from the edge of a canal, and, like them, become more and more faint as they flow on, one after another, till they melt into the general level of the internal temperature. The parts of the earth near the equator,” added the old man, “are more heated by the sun than other parts, and on this account there is a perpetual internal _conduction_ of heat from the equatorial to the northern and southern regions. Then, as all parts of the earth’s surface throw off heat into space by radiation, it is plain that at the poles, where the surface receives but little warmth from the sun, a constant waste of caloric is produced. There is thus a perpetual dispersion of heat _from the polar parts_ into surrounding space, which is supplied by a perpetual internal flow of heat _from the equator_ towards the poles. The radiation from the surface of the earth,” Mr. Tonkin concluded, “has its limit in the temperature of the planetary space in which it moves (for we may conceive our globe to be like a heated ball cooling down, _in vacuo_), and this has been calculated to be not more than 56° below zero,—which low temperature, indeed, appears to be attained in the long absence of the sun in a polar winter.”

The poetic boy was lost in wonder at the marvellous results to which his investigations had led him, and his mind was filled with a sense of sublimity at the thought of the enormous heat-tides that are continually flowing through the atmosphere, the ocean, and the solid crust of the earth itself.

“I’ll work it all out myself,” he cried; “that I will. I’ll not rest until I know all that is known of Nature and her wondrous ways.”