Imágenes de páginas

much farther than was generally supposed,—probably, he thought, even to the moon . The result we must state in Dr. Brewster's language, which it will be necessary for the reader to peruse with some attention, as from the nature of the subject it may not appear at first quite so ree from obscurity, as upon a little reflection he will find it.

‘That her motion must be influenced by such a power, he did not for a moment doubt; and a little reflection convinced him that it might be sufficient for retaining that luminary in her orbit round the earth. Though the force of gravity suffers no sensible diminution at those small distances from the earth's centre at which we can place ourselves, yet he thought it very possible, that, at the distance of the moon, it might differ much in strength from what it is on the earth. In order to form some estimate of the degree of its diminution, he considered, that, if the moon be retained in her orbit by the force of gravity, the primary planets must also be carried round the sun by the same power; and by comparing the periods of the different planets with their distances from the sun, he found, that, if they were retained in their orbits by any power like gravity, its force must decrease in the duplicate proportion,” or as the squares of their distances from the sun. In drawing this conclusion, he supposed the planets to move in orbits perfectly circular, and having the sun in their centre. Having thus obtained the law of the force by which the planets were drawn to the sun, his next object was to ascertain if such a force emanating from the earth, and directed to the moon, was sufficient, when diminished in the duplicate ratio of the distance, to retain her in her orbit. In performing this calculation, it was necessary to compare the space through which heavy bodies fall in a second at a given distance from the centre of the earth, viz. at its surface, with the space through which the moon, as it were, falls to the earth in a second of time while revolving in a circular orbit. Being at a distance from books when he made this computation, he adopted the common estimate of the earth's diameter then in use among geographers and navigators, and supposed that each degree of latitude contained sixty English miles. In this way he found that the force which retains the moon in her orbit, as deduced from the force which occasions the fall of heavy bodies to the earth's surface, was one-sixth greater than that which is actually observed in her circular orbit. This difference threw a doubt upon all his speculations; but, unwilling to abandon what seemed to be otherwise so plausible, he endeavoured to account for the difference of the two forces, by supposing that some other cause + must have been united with the force of gravity in producing so great a velocity of the moon in her circular orbit. As this new cause, however, was beyond the reach of observation, he discontinued all farther inquiries into the subject, and concealed from his friends the speculations in which he had been employed.

“After his return to Cambridge in 1666, his attention was occupied with

* “But for the duplicate proportion, I gathered it from Kepler's theorem about twenty years ago.”—Newton's Letter to Halley, July 14th, 1686.

+ “Whiston asserts that this cause was supposed by Newton to be something analogous to the vortices of Descartes.—See Whiston's Memoirs of Himself, p. 231.”

those optical discoveries of which we have given an acconnt in a preceding chapter; but he had no sooner brought them to a close than his mind reverted to the great subject of the planetary motions. Upon the death of Oldenburg in August, 1678, Dr. Hooke was appointed secretary to the Royal Society; and as this learned body had requested the opinion of Newton about a system of physical astronomy, he addressed a letter to Dr. Hooke on the 28th November, 1679. In this letter he proposed a direct experiment for verifying the motion of the earth, viz. by observing whether or not bodies that fall from a considerable height descend in a vertical direction, for if the earth were at rest the body would describe exactly a vertical line, whereas if it revolved round its axis, the falling body must deviate from the vertical line towards the east. The Royal Society attached great value to the idea thus casually suggested, and Dr. Hooke was appointed to put it to the test of experiment. Being thus led to consider the subject more attentively, he wrote to Newton, that wherever the direction of gravity was oblique to the axis on which the earth revolved, that is, in every part of the earth except the equator, falling bodies should approach to the equator, and the deviation from the vertical, in place of being exactly to the east, as Newton maintained, should be to the south-east of the point from which the body began to move. Newton acknowledged that this conclusion was correct in theory, and Dr. Hooke is said to have given an experimental demonstration of it before the Royal Society in December, 1679.” Newton had erroneously concluded, that the path of the falling body would be a spiral; but Dr. Hooke, on the same occasion on which he made the preceding experiment, read a paper to the Society in which he proved that the path of the body would be an eccentric ellipse in vacuo, and an ellipti-spiral, if the body moved in a resisting medium.t ‘This correction of Newton's error, and the discovery that a projectile would move in an elliptical orbit when under the influence of a force varying in the inverse ratio of the square of the distance, led Newton, as he himself informs us in his letter to Halley,t to discover “the theorem by which he afterwards examined the ellipsis,” and to demonstrate the celebrated proposition, that a planet acted upon by an attractive force varying inversely as the squares of the distances, will describe an elliptical orbit in one of whose foci the attractive force resides. “But though Newton had thus discovered the true cause of all the celestial motions, he did not yet possess any evidence that such a force actually resided in the sun and planets. The failure of his former attempt to identify the law of falling bodies at the earth's surface with that which guided the moon in her orbit, threw a doubt over all his speculations, and prevented him from giving any account of them to the public. ‘An accident, however, of a very interesting nature induced him to resume his former enquiries, and enabled him to bring them to a close. In June, 1682, when he was attending a meeting of the Royal Society of London, the measurement of a degree of the meridian, executed by M. Picard, in 1679, became the subject of conversation. Newton took a memorandum of the result obtained by the French astronomer, and having

[ocr errors]

deduced from it the diameter of the earth, he immediately resumed his calculation of 1665, and began to repeat it with these new data. In the progress of the calculation he saw that the result which he had formerly expected was likely to be produced, and he was thrown into such a state of nervous irritability that he was unable to carry on the calculation. In this state of mind he intrusted it to one of his friends, and he had the high satisfaction of finding his former views amply realized. The force of gravity which regulated the fall of bodies at the earth's surface, when diminished as the square of the moon's distance from the earth, was found to be almost exactly equal to the centrifugal force of the moon as deduced from her observed distance and velocity.

“The influence of such a result upon such a mind may be more easily conceived than described. The whole material universe was spread out before him:-the sun with all his attending planets;–the planets with all their satellites;-—the comets wheeling in every direction in their eccentric orbits;—and the systems of the fixed stars stretching to the remotest limits of space. All the varied and complicated movements of the heavens, in short, must have been at once presented to his mind as the necessary result of that law which he had established in reference to the earth and the moon.’—pp. 148–153.

Such was the origin of the celebrated Principia of Newton, a work which Dr. Brewster truly describes as ‘memorable not only in the annals of one science or of one country, but which will form an epoch in the history of the world, and will ever be regarded as the brightest page in the records of human reason.’ He then proceeds to give an analysis of the three books of which it is composed, the first and second being devoted to the “motion of bodies” in general, the third to the “system of the world.” Important as were the subjects comprehended in this treatise, simple as was the system which it unfolded, and unanswerable as was the reasoning and irresistible the evidence by which that system was supported, yet was it scarcely ushered into the world, when it met with such a tide of prejudice, that Newton almost again repented that he had parted with “so substantial a blessing as his quiet, to run after a shadow.” His whole theory was pulled to pieces, and of those astronomers who did not oppose it, several claimed particular portions of the new doctrine as their own. This, cried one, is “my thunder,” and this, cried a second and a third, is mine; and so they went on, each stripping Newton of something which they contended did not belong to him, until it became evident to all men that the great perambulator of the Heavens had really made no discovery at all ! Such is uniformly the fate of the most distinguished minds. Assailed by prejudice, underrated by insolent ignorance, they seldom enjoy in their life-time the reputation which is, in most cases, the only reward they can obtain for their labours. For Newton, however, a happier fate was reserved. His existence was prolonged beyond the ordinary period assigned to men, as if for the express purpose of affording him that grateful compensation for his noble toils; and he had the satisfaction, before his death, of seeing his philosophy not only ascribed to its real author, but triumphant over all its opponents. The mathematical writings of Newton, and the celebrated controversy respecting the invention of fluxions, afford to Dr. Brewster materials for a chapter, which, though highly interesting in a scientific point of view, we must pass over, in order to make room for some notice of a passage in the personal history of the great astronomer, which has been much misrepresented by M. Biot. The misrepresentation has already been exposed in this Journal (vol xi. for 1829, p. 591): but Dr. Brewster has placed the vindication of Newton's sanity in a still more satisfactory point of view, in consequence of the hitherto unpublished evidence which he has been enabled to produce. The story of the destruction of some papers by the overturning, by his dog Diamond, of a candle which he had left burning in his room, while he attended chapel one morning in winter, is well known, and the loss thus supposed to have been incurred, is said by M. Biot, upon the authority of a note found among the manuscripts of Huygens, to have so much distressed the mind of Newton as to have removed it for a season from its throne. This event is alleged to have occurred towards the close of the year 1692; but Dr. Brewster has given an extract from a diary kept by Mr. de la Pryme, who was a student in Cambridge in 1692, from which it appears that the accidental destruction of the papers in question must have taken place previously to the 3d of January in that year. The statement in de la Pryme’s Diary is as follows:— * “1692, February 3d.—What I heard to-day I must relate. There is one Mr. Newton, (whom I have very oft seen,) Fellow of Trinity College, that is mighty famous for his learning, being a most excellent mathematician, philosopher, divine, &c. He has been Fellow of the Royal Society these many years; and amongst other very learned books and tracts he's written one upon the mathematical principles of philosophy, which has got him a mighty name, he having received, especially from Scotland, abundance of congratulatory letters for the same; but of all the books that he ever wrote, there was one of colours and light, established upon thousands of experiments which he had been twenty years of making, and which had cost him many hundred of pounds. This book, which he valued so much, and which was so much talked of, had the ill luck to perish, and be utterly lost just when the learned author was almost at putting a conclusion at the same, after this manner: In a winter's morning, leaving it amongst his other papers on his study table whilst he went to chapel, the candle, which he had unfortunately left burning there too, catched hold by some means of other papers, and they fired the aforesaid book, and utterly consumed it and several other valuable writings; and, which is most wonderful, did no further mischief. But when Mr. Newton came from chapel, and had seen what was done, every one thought he would have run mad, he was so troubled thereat, that he was not himself for a month after. A long account of this his system of light and colours you may find in the Transactions of the Royal Society, which he had sent up to them long before this sad mischance happened unto him.’”—pp. 228, 229.


The phrase that “every one thought he would have run mad,” proves, as Dr. Brewster justly remarks, that no such effect was produced, and the addition that “he was so troubled thereat that he was not himself for a month after,” merely indicates that distress of mind which might be sufficient to prevent a person from attending to his occupations with his wonted assiduity and cheerfulness for a while, and nothing more. It cannot, however, now be doubted, that there was some ground for the report that was conveyed to Huygens, the extent of which, the letters supplied by Lord Braybrooke have enabled Dr. Brewster to ascertain. The first of these letters, dated the 13th of September, 1693, is addressed by Newton himself to Mr. Pepys, then secretary to the Admiralty.

• “Sir,

“Some time after Mr. Millington had delivered your message, he pressed me to see you the next time I went to London. I was averse; but upon his pressing consented, before I considered what I did, for I am extremely troubled at the embroilment I am in, and have neither ate nor slept well this twelvemonth, nor have my former consistency of mind. I never designed to get any thing by your interest, nor by King James's favour, but am now sensible that I must withdraw from your acquaintance, and see neither you nor the rest of my friends any more, if I may but leave them quietly. I beg your pardon for saying I would see you again, and rest your most humble and most obedient servant,

* “Is. NEwton.’”—p. 232.

Whatever Newton may have meant here by the usual consistency of his mind, it is very well known that it was during the twelvemonth thus referred to, he wrote his four celebrated letters to Dr. Bentley, on the Existence of a Deity. Mr. Millington was a fellow of Magdalene College, and the mention of him is supposed to have been connected with an application that was made by some of Newton's friends to the government, for an appointment that might render his circumstances more comfortable. The letter produced a strong impression upon Pepys, at which we do not wonder, for it must be confessed that it contains several awkward expressions; he very naturally requested from his friend Millington further explanations, as to Newton's actual condition, stating that he had lately received a letter from him “so surprizing to me for the inconsistency of every part of it, as to be put into great disorder by it, from the concernment I have for him, lest it should arise from that, which of all mankind I should least dread from him and most lament for, I mean discomposure in head, or mind, or both.” Millington's answer is satisfactory, and really places the whole affair upon its true foundation. - - Coll. Magd. Camb.

““HoNor’D SIR, - Sept. 30, 1693.

* “Coming home from a journey on the 28th instant at night, I met with your letter which you were pleased to honour me with of the 26th. I


« AnteriorContinuar »