Part 19

it may be said that to him we owe, in some degree, the publication of the ‘Principia;’ for Halley being engaged upon the consideration of Kepler’s law, as it had been discovered by observation, viz., that the squares of the periodic times of planets are as the cubes of their distances, and suspecting that this might be accounted for on the supposition of a centripetal force, varying inversely as the square of the distance, applied himself to prove the connexion geometrically, in which he was unable to succeed. In this difficulty he applied to Hook and Wren, neither of whom could help him, and was recommended to consult Newton, then Lucasian Professor at Cambridge. Following this advice, he found in Newton all he wanted; and did not rest until he had persuaded his new acquaintance to give the results of his discoveries to the world. In about two years after this, the first edition of the ‘Principia’ was published, and the proofs were corrected by Halley, who supplied the well-known Latin verses which stand at the beginning of the work.

In conversation, Halley appears to have been of a jocose and somewhat satirical disposition. The following anecdote of him, which is told by Whiston, displays the usual modesty of the latter, when speaking of himself: “On my refusal from him of a glass of wine on a Wednesday or Friday, he said he was afraid I had a pope in my belly, which I denied, and added somewhat bluntly, that had it not been for the rise now and then of a Luther or a Whiston, he would himself have gone down on his knees to St. Winifred or St. Bridget, which he knew not how to contradict.” It is related that when Queen Caroline offered to obtain an increase of Halley’s salary as Astronomer Royal, he replied, “Pray, your Majesty, do no such thing, for should the salary be increased, it might become an object of emolument to place there some unqualified needy dependant, to the ruin of the institution.” And yet the sum which he would not suffer to be increased was only £100 a-year.

To give even a catalogue of the various labours of Halley, would require more space than we can here devote to the subject. For a more detailed account both of his life and discoveries, we must refer the reader to the Biographia Britannica, to Delambre, Histoire de l’Astronomie au dix-huitième Siecle, livre II., and the Philosophical Transactions of the time in which he lived; or better perhaps to the Miscellanea Curiosa, _London_, 1726, a selection of papers from the Transactions, containing the most remarkable of those written by Halley. We shall, nevertheless, proceed briefly to notice a few of the discoveries on which the fame of our astronomer is built.

The most remarkable of them, to a common reader, is the conjecture of the return of a comet. Some earlier astronomers, as Kepler, had imagined the motion of these bodies to be rectilinear. Newton, in explaining the principle of universal gravitation, showed how a comet might describe a parabola, and also how to calculate its motion, and compare it with observation. Hevelius had already indicated the curvature of a comet’s path, and Dörfel, a Saxon clergyman, had calculated the path of the comet of 1680 upon this supposition. Halley, in computing the parabolic elements of all the comets which had been well observed up to his time, suspected, from the general likeness of the three, that the comets of 1531, 1607, and 1682, were the same. He was the more confirmed in this, by knowing that comets had been seen, though no good observations were recorded, in the years 1305, 1380, and 1456, giving, with the former dates, a chain of differences of 75 and 76 years alternately. Halley supposed, therefore, that the orbit of this comet was, not a parabola, but a very elongated ellipse, and that it would return about the year 1758. The truth of his conjecture was fully confirmed in January, 1759, by Messier. The first person, however, who saw Halley’s comet, as it is now called, was George Palitzch, a farmer in the neighbourhood of Dresden, who had studied astronomy by himself, and fitted up a small observatory.

But a much more useful exertion of Halley’s genius and power of calculation is to be found in his researches on the lunar theory. It is to him that we are indebted for first starting the idea of finding the longitude at sea by means of the moon’s place, which is now universally adopted. The principle of this problem is as follows. An observer at sea can readily find the time of day by means of the sun or a star, and can thereby correct a watch. If he could at the same moment in which he finds his own time, also discover that at Greenwich, the difference between the two, turned into degrees, minutes, and seconds, would be his longitude east or west of Greenwich. If, therefore, he carries with him a Nautical Almanac, in which the times of various astronomical phenomena are registered, as they will take place at Greenwich, or rather as they will be seen by an observer placed at the centre of the earth with a Greenwich clock, he can observe any one of these phenomena, and reduce it also to the centre. He will then know the corresponding moments of time, for his own position and that of Greenwich. The moon traverses the whole of its orbit in little more than 27 days, and therefore moves rapidly with respect to the fixed stars, its motion being nearly a whole sign of the zodiac in 48 hours. If we observe the distance between the moon and a star, and find it to be ten degrees, the longitude of the place in which the observation is made can be known as aforesaid, if the almanac will tell what time it was at Greenwich when the moon was at that same distance from the star. In the time of Halley, though it was known that the moon moved nearly in an ellipse, yet the elements of that ellipse, and the various irregularities to which it is subject, were very imperfectly ascertained. It had, however, been known even from the time of the Chaldeans, that some of these irregularities have a _period_, as it is called, of little more than eighteen years, that is, begin again in the same order after every eighteen years; the periods and quantities of several other errors had also been discovered with something like accuracy. To make good lunar tables, that is, tables from which the place of the moon might be correctly calculated beforehand, became the object of Halley’s ambition. He therefore observed the moon diligently during the whole of one of the periods of eighteen years, that is, from the end of 1721 to that of 1739, and produced tables which were published in 1749, after his death, and were of great service to astronomers. He also made another observation on the motion of the moon, which has since given rise to one of the finest discoveries of Laplace. In calculating from our tables the time of an ancient eclipse, observed at Babylon, B. C. 720, he found that, had the tables been correct, it would have happened three hours sooner than, according to Ptolemy, it did happen. This might have arisen from an error in the Babylonian observation; but on looking at other eclipses, he found that the ancient ones always happened later than the time indicated by his table, and that the difference became less and less as he approached his own time. From hence he concluded that the moon’s average daily motion is subject to a very small acceleration, so that a lunar month at present is in a very slight degree shorter than a month in the time of the Chaldeans. This was afterwards shown by Laplace to arise from a very slow diminution in the eccentricity of the earth’s orbit, caused by the attraction of the planets. For a further account of Halley’s astronomical labours, we may refer to the History of Astronomy in the Library of Useful Knowledge, page 79.

We must also ascribe to Halley the first correct application of the barometer to the measurement of the heights of mountains. Mariotte, who first enunciated the remarkable law that the elastic forces of gases are in the inverse proportion of the spaces which they occupy, had previously given a formula for the determination of these same heights, entirely wrong in principle, and inapplicable in practice. Halley, whose profound mathematical knowledge made him fully equal to the task, investigated and discovered the common formula, which, with some corrections for the temperature of the mercury in the barometer and the air without it, is in use at this day. We have already mentioned that Halley sailed to various parts of the earth with a view to determine the variation of the magnet. The result of his labours was communicated to the Royal Society in a map of the lines of equal variation, and also of the course of the trade-winds. He attempted to explain the phenomena of the compass by supposing that the earth is one great magnet, having four poles, two near each pole of the equator; and further accounts for the variation which the compass undergoes from year to year in the same place, by imagining a magnetic sphere, interior to the surface of the earth, which nucleus or inner globe turns on an axis with a velocity of rotation very little differing from that of the earth itself. This hypothesis has shared the fate of many others purely mathematical; that is, invented to show how the observed phenomena might be produced, without any ground of observation for believing that they really are so produced. If we put together the astronomical and geographical discoveries of Halley, and remember that the former were principally confined to those points which bear upon the subjects of the latter, we shall be able to find a title for their author less liable to cavil than that of the Prince of Astronomers, which has sometimes been bestowed upon him; we may safely say that no man, either before or since, has done more to improve the theoretical part of navigation, by the diligent observation alike of heavenly and earthly phenomena.

We pass over many minor subjects, such as his improvement of the diving-bell, or his measurement of the quantity of fluid abstracted by evaporation from the sea, to come to an application of science in which he led the way,—the investigation of the law of mortality. From observations communicated to the Royal Society of the births and deaths in the city of Breslau, he constructed the first table of mortality, which was in a great measure the foundation of the celebrated hypothesis of De Moivre, that the decrements of human life are nearly equal at all ages; that is, that out of eighty-six persons born, one dies every year, until all are gone. Halley’s table as might be expected, was not very applicable to human life in England, either then or now, but the effect of example is conspicuous in this instance. Before the death of Halley the tables of Kerseboom were published, and four years afterwards, those of De Parcieux.

We will not enlarge on the purely mathematical investigations of Halley, which would possess but little interest for the general reader. We may mention, however, his method for the solution of equations, his ‘Analogy of the Logarithmic Tangents to the Meridian Line, or sum of the secants,’ his algebraic investigation of the place of the focus of a lens, and his improvement of the method of finding logarithms. From the latter we quote a sentence, which, to the reader, for whose benefit we have omitted entering upon any discussion of these subjects, will appear amusing enough, if indeed he does not shrink to see how much he has degenerated from his ancestors. After describing a process which contains calculation enough for most people; and which further directs to multiply sixty figures by sixty figures, he adds, “If the curiosity of any gentleman that has leisure, would prompt him to undertake to do the logarithms of all prime numbers under 100,000 to 25 or 30 figures, I dare assure him that the facility of this method will invite him thereto; nor can anything more easy be desired. And to encourage him, I here give the logarithms of the first prime numbers under 20 to 60 places.” One look at these encouraging rows of figures would be sufficient for any but a calculating boy.

No one who is conversant with the mathematics and their applications can read the life of the mathematicians of the seventeenth century without a strong feeling of respect for the manner in which they overcame obstacles, and of gratitude for the labour which they have saved their successors. The brilliancy of later names has, in some degree, eclipsed their fame with the multitude; but no one acquainted with the history of science can forget, how with poor instruments and imperfect processes, they achieved successes, but for which Laplace might have made the first rude attempts towards finding the longitude, and Lagrange might have discovered the law which connects the coefficients of the binomial theorem. But even of these men the same thing may one day be said; and future analysts may wonder how Laplace, with his paltry means of investigation, could account for the phenomenon of the acceleration of the moon’s motion; and future astronomers may, should such a sentence as the present ever meet their eyes, be surprised that the observers of the nineteenth century should hold their heads so high above those of the seventeenth.

[Illustration:

_Engraved by W. Holl._

SULLY.

_From the original Picture by an unknown Artist in the private collection of Louis Philippe, King of the French._

Under the Superintendance of the Society for the Diffusion of Useful Knowledge

_London. Published by Charles Knight, Pall Mall East._ ]

[Illustration]

SULLY

The Duc de Sully is celebrated as the companion, minister, and historian of Henry IV., the most popular of French monarchs. Eminent among his contemporaries both as a soldier and as a financier, it is his especial glory that he laboured to promote the welfare of the industrious classes, when other statesmen regarded them but as the fount from which royal extravagance was to be supplied.

Maximilian, son of François de Bethune, Baron de Rosny, and of Charlotte Dauvet, daughter of a President of the Chamber of Accounts at Paris, was born at Rosny in the year 1559. His family was ancient, illustrious, and once wealthy, but his paternal grandfather had almost ruined it by his extravagance, his maternal grandfather disinherited him because he embraced the reformed religion; and with a slight annual allowance young Rosny had to seek his own fortune in the extravagant profession of arms. By a sage economy and order he, however, supported himself, and escaped the dependence and dishonour consequent on extravagance in a poor man. When thirteen years of age he was presented by his father to the young Prince of Navarre, who was only seven years older than himself, and who at once conceived that affection for him which was destined to cease only with his own life.

On the memorable day of St. Bartholomew, Rosny was in Paris, engaged in the prosecution of his studies. A known member of the Protestant Church, his life was in jeopardy: his servant and his tutor fell victims to the rage of the Papists, and he himself, obliged to quit his chambers for a safer hiding-place, and exposed to imminent dangers in traversing the streets, owed his deliverance more than once to a union of courage and coolness not very common in a youth of thirteen. After this event he, as well as his patron and friend Henry of Navarre, conformed for a time to the observances of the Roman Catholic religion; but in 1576, when Henry escaped from the thraldom in which he had been held, abjured Catholicism and placed himself at the head of a Protestant army, Rosny was the companion of his flight, and first began to carry arms in his service. His noble birth, and the favour of his master, would at once have secured him military rank, but Rosny preferred to serve as a simple volunteer, in order, as he said, to learn the art of war by its elements.

At the surprise of Réde, at the siege of Villefranche, at the taking of Eause and Cahors, at the battle of Marmande, and in all the dangerous affairs in which Henry engaged, Rosny was always at his side. His good services, and the affection borne him by his master, did not, however, prevent a quarrel, which, it must be said, was provoked by his own imprudence and aggravated by his own pride. In spite of the commands of the Prince of Navarre, who had wisely prohibited the practice of referring private quarrels to the arbitrement of the sword, Rosny acted as second in a duel, in which one of the principals was desperately wounded. The Prince’s anger at the breach of discipline was exasperated by a strong personal regard for the wounded man. He sent for Sully, rebuked him in harsh terms, and said that he deserved to lose his head for what he had done. The pride of the young soldier was touched; he replied that he was neither vassal nor subject of Navarre, and would henceforth seek the service of a more grateful master. The Prince rejoined in severe terms and turned his back on him; and Rosny was quitting the court, when the Queen, who knew his value, interfered, and reconciled him with her son.

Not long after he quitted Henry’s service, alleging that he had pledged his word to accompany the Duc d’Alençon, afterwards Duc d’Anjou, brother of Henry III., in his contest for the sovereignty of Flanders; where, in case of success, he was to be put in possession of the estates which had belonged to his maternal grandfather. In this campaign he gained neither honour nor profit, and soon returned to his original master. Henry received him with open arms, and, as if to prove that absence had not affected his confidence and esteem, sent him a few days after on an important mission to Paris.

In the troubled times which followed, Rosny was unshaken in devotion to the cause which he had espoused. He accompanied Henry, when that prince, with only nineteen followers, threw himself, as a last resource, into La Rochelle. He undertook an embassy from that city to Henry III., then almost as much persecuted by the League as the King of Navarre himself. In his Memoirs he has left a striking description of the degraded condition of that sovereign, who had entirely abandoned himself to favourites and menials of the court. “His Majesty was in his cabinet; he had his sword by his side, a hood thrown over his shoulders, a little bonnet on his head, and a basket full of little dogs hung round his neck by a broad riband.” He listened to Rosny with vacant stupidity, neither moving his feet, his hands, nor his head. When he spoke, he complained of the audacity and insults of the League—said that nothing would go well in France until the King of Navarre went to mass—but agreed, finally, that Rosny might treat with the envoys of the Protestant Cantons of Switzerland, in his name as well as the King of Navarre’s, for the raising of twenty thousand Swiss troops, to be employed between the two sovereigns.

Henry, through his imprudence, lost all the advantages which his faithful servant’s treaty with the Swiss might have secured to him; but neither disgusted nor dispirited by this folly, Rosny persevered in his attachment to a cause which seemed altogether desperate to most others. He was at the siege of Fontenay, and at the brilliant victory of Coutras, for which the King of Navarre was materially indebted to the artillery under Rosny’s command. His next great undertaking was to effect an entire reconciliation between his master and the King of France. Having succeeded in this, the eyes of all France thenceforward rested upon him as the only man who could re-establish the distracted kingdom. Such was the enthusiasm of many of the French at the time, that they called him “Le Dieu Rosny.”

The desired reconciliation had not long been made when Henry III. was assassinated by a fanatic monk, and the King of Navarre laid claim to the vacant throne. But much remained to be done ere he could tranquilly seat himself upon it. His religion was an insurmountable obstacle to the mass of the nation, and the League was all-powerful in many parts of France and held possession of Paris.

Rosny fought with his accustomed valour at the battles of Arques and Ivry. At the latter he well nigh lost his life: he received five wounds, had two horses killed under him, and fell at last among a heap of slain. The manner in which he retired from this field, with four prisoners of the highest distinction and the standard of the enemy’s commander-in-chief, is one of the most romantic incidents to be found in authentic history.

After the victory of Ivry, Rosny did not receive the rewards he merited, and he remained for some time at his estate under pretence of ill health, but secretly disinclined to return to the service of one who had shown little real gratitude for his long and faithful adherence. No sooner, however, did he learn that Henry was about to undertake the siege of Paris, than he left his retreat and hastened again to his master’s side. His wounds were still uncured: he appeared before the King leaning on crutches and with an arm in a sling. Touched by his devotedness and his melancholy state, Henry loaded him with caresses, and insisted that he should not expose himself for the present but remain near his person to assist him with his counsels.