CHAPTER LI.
Section 275. If then this Reasoning be allowed; aërial Travellers will not be subject, when, at a considerable Height, even in Winter, to great Degrees of Cold, supposing that the Air does not actually freeze the Waters below; and the Apogay or Southerly Winds have continued for a few Days.
On the Contrary; Aironauts may expect Cold, encreasing with their Ascent, even in Summer, tho’ _warm_ below; supposing the Eknèfiai or Northerly Winds to have continued but for a Day before the Ascent: they may possibly, indeed by soaring higher, rise into the regular Stratum of the warm Apogay floating above them.
276. From what has been said, there seems a Degree of Probability, that the Air for a Number of Miles, _above warm cultivated Plains_ shoud differ materially in its Temperature, from Air above Mountains, or _even on a Level_ with their Summits.
That the former Air, in moderate Weather, shoud continue _warm and rarefied_: while the latter is _cool and condensed_.
For the same Reason the Air over the Sea, on the Hours of Accumulation; i.e. during the Night, in Summer, and frequently in Winter, shoud be found _warm_ and _rarefied_: especially during a Continuance of the Apogay Winds.
277. It is likewise probable that the Atmosphere will be found +respirable+ at much greater Heights, than is at present imagined: during the Continuance of the Eknèfiai Winds; and also, on Account of the _defloguisticated_ Air,[83] which is _drier_ and _less elastic_ in Proportion to its Rarity.[84]
278. The Height of 10 Miles seems not too great to limit human Respiration, shoud any Attempt be made, to soar with a Balloon in a mild Atmosphere; and particularly between the Tropics.[85]
But an Objection woud be found in the Size of a Balloon sufficiently capacious to contain nearly 6 Times the Bulk to which the Gass woud necessarily expand itself, at the Height of 10 Miles.
[Sidenote: First Cause of Limitation, in the Ascent of Balloons.]
[Sidenote: Second Cause of Limitation in the Ascent of Balloons.]
279. It seems most likely that the primary Cause that will affect the Ascent of Balloons is the Difficulty of encreasing the Dimension of the Balloon: the Second, is from the excessive Cold; if the Wind blows from any Points of the North.
Supposing the Construction of the Atmosphere to be as represented by different Authors, (which, by the Way, is scarcely credible) ten Miles will perhaps be the utmost attainable Height.
280. There is a Circumstance relative to the Motion of the Air, which has not been sufficiently attended to: and bears some Analogy with that of a _Thorough Air_.
This Circumstance may not improperly be called the _Reception_ and _Dispersion of Air_.
In cold Climates, it is an Object of Dread: in warm ones, a most desirable Piece of Luxury.
A gentle Undulation of the Air is perceived in Peru, and other hot Climates, by Persons sitting in _Arbours_ sheltered from the Sun.
The surrounding Air is instantly _contracted_ by _Condensation_, during the Absence of the Sun’s Rays, and therefore occupies a _less Space_: _fresh Air_ is _received_, and as instantly _dispersed_ by Expansion towards those Parts, which are the warmest, i.e. where there is least Resistance: so that a gentle Breeze is constantly kept up, _probably_ by a Depression from _above_.[86]
281. Analagous to this, are those Winds which generally _rise early_ and die away at _Sunset_: the nocturnal Condensation of the Air being sufficient for the +reception+: as Air suffers some Compression without Tumult.
To demonstrate the Changes owing also to remote and invisible Causes least suspected; Boyle somewhere speaks of an Instrument he made, which was so nicely contrived, that he coud tell, while sitting in his own Apartment, whenever any detached Cloud passed beneath the Sun’s Disk. The Principle on which it acted seems to have been that of a Reception and Dispersion of Air that took Place within _the_ +shadow+ proceeding from the Cloud.
282. An oblique Argument supporting the Doctrine of Depression, asserted to take Place, in fair Weather, is that _Wind_ drys up the Moisture from the Ground more than the _Sun_: and that March which is the _windiest_, is also the _most drying_, tho’ _not_ the _hottest_ Month.
Bacon, in his Enquiry into Motions and Undulations of the Air, uses a Metaphor, which tho’ somewhat facetious, is strictly philosophical.[87] “_For when_ +winds+ _lead_ +the dance+, _it woud be agreeable to know the_ +figure+.”[88]
And it is probable, that they really press the Earth with a saltatory progressive undulating Motion, _descending_ in elastic Steps of sudden Compression; and _rising_ with quick alternate ones, of Dilatation and Expansion.
Dicker’s Balloon gave Proof of this.
283. Lastly: the +chill+ _of Air_ which always takes Place over +water+, and _moist_ Grounds, even in the +finest weather+, strongly favours the _Reception_ and _Dispersion_ of _it_, to the surrounding and more heated Lands: (which can only be supplied, as before mentioned, by Torrents of fresh Air _gradually descending_ from the etherial or middle Region of the Atmosphere;) and seems to produce the same Effect, viz. a constant Breeze, with that of the Arbor, Shade, or Shelter from the _Sun_: also with that of the _Shadow_ from the Cloud passing under his Disk, which affected a complete Thermometer and Hygrometer.
284. On a Change of Weather from Frost to Thaw, the Colour of the _upper Air_ +first+ alters from a _clear and deep_, to a _dull and faint_ Blue, or to a muddy Haze, not distinguishable into Clouds, but visible above them; a vivid Brightness still remaining, for many Hours, to about 500 Yards above the Surface of the Earth.
Or, soft _warm_ Showers fall gently, without Wind, or any apparent Change in its Direction.
All which seem to favour the Accumulation and Descent of _warm Air_, by Waves of the Apogay rolling over the Eknèfiai Winds.