that time, any farther thoughts upon this matter. But some years after, a letter, which he received from Dr. Hooke, put him on inquiring what was the real figure, in which a body let fall from any high place descends, taking the motion of the earth round its axis into consideration. Such a body, having the same motion, which by the revolution of the earth the place has whence it falls, is to be considered as projected forward and at the same time drawn down to the centre of the earth. This gave occasion to his resuming his former thoughts concerning the moon, and Picard in France having lately measured the earth, by using his measures the moon appeared to be kept in her orbit purely by the power of gravity; and consequently, that this power decreases, as you recede from the centre of the earth, in the manner our author had formerly conjectured. Upon this principle he found the line described by a falling body to be an ellipsis, the centie of the earth being one focus. And the primary planets moving in such orbits round the sun, he had the satisfaction to see, that this inquiry, which he had undertaken merely out of curiosity, could be applied to the greatest purposes. Hereupon he composed near a dozen propositions, relating to the motion of the primary planets about the sun. Several years after this, some discourse he had with Dr. Halley, who at Cambridge made him a visit, engaged Sir Isaac Newton to resume again the consideration of this subject; and gave occasion to his writing the treatise, which he published under the title of Mathematical Principles of Natural Philosophy. This treatise, full of such a variety of profound inventions, was composed by him, from scarce any other materials than the few propositions before mentioned, in the space of a year and a half. No. XIII. AUBREY AND HOOKE'S LETTER Mr. Wood! Sept. 15, 1689. Mr. Rob. Hooke, R. S. S., did in anno 1670 write a discourse called, An Attempt to prove the Motion of the Earth, which he then read to the Royal Society; but printed it in the beginning of the year 1674. . . . to Sir John Cutler, to whom it is dedicated, wherein he has delivered the theory of explaining the celestial motions mechanically; his words are these, pag. 27, 28, viz. ". About 9 or 10 years ago Mr. Hooke writ to Mr. Isaac Newton of Trin. Coll. Cambridge, to make a demonstration of [it] this Theory, not telling him at first the proportion of the gravity to the distance, [and] nor what was the curved line that was thereby made. X Mr. Newton [did express], in his answer to the letter, did express that he had not thought of it; and in his first attempt about it, he calculated the curve by supposing the attraction to be the same at all distances: upon which Mr. Hooke told him in his s In printing the following letter whatever was written by Aubrey is in the Roman, whatever was inserted by Hooke is in the Italic types. Aubrey's words are inclosed in brackets when they have been erased in order for others to be substituted for them. t The words here are not legible, but probably they make mention of Hooke " as lecturer to Sir John Cutler," or something to that purport. u Here a space is left in which Aubrey evidently intended to insert the passage. 66 X "known" is written over thought;" but the first word is not erased. next letter the whole of his Hypothesis, scil. that the gravitation was reciprocal to the square of the distance, which would make the motion in an ellipsis, in one of whose foci the sun being placed, the aphelion and perihelion of the planet would be opposite to each other in the same line, which is the whole celestial theory, concerning which Mr. Newton hath made a demonstration, not at all owning he received the first intimation of it from Mr. Hooke. Likewise Mr. Newton has in the same book printed some other theories and experiments of Mr. Hooke's, as that about the oval figure of the earth and sea: without acknowledging from whom he had [it] them, though he had not sent it up with the other parts of his book, till near a month after this theory was read to the Society by R. H., (Mr. Hooke,) when it served to help to answer Dr. Wallis his arguments produced in the R. S. against it. In the Attempt to prove the Motion of the Earth, &c. printed 1674, but read to the Royal Society 1671, pag. 27, lin. 31. "I shall only for the present hint, that I have, in "some of my foregoing observations, discovered some "new motions even in the earth itself, which perhaps 66 66 66 66 were not dreamt of before, which I shall hereafter "more at large describe, when further trials have more fully confirmed and completed these beginnings. At which time also I shall explain a system of the world, differing in many particulars from any yet known, answering in all things to the com"mon rules of mechanical motions. This depends 66 upon three suppositions. First, that all celestial "bodies whatsoever have an attraction or gravitating power towards their own centres, whereby they at"tract not only their own parts and keep them from 66 "flying from them, as we may observe the earth to 66 66 66 66 66 66 66 66 66 do, but that they do also attract all the other ce"lestial bodies, that are within the sphere of their activity, and consequently that not only the sun and "the moon have an influence upon the body and mo"tions of the earth, and the earth upon them, but that Mercury also, Venus, Mars, Saturn, and Jupiter, by their attractive powers have a considerable influence upon its motion, as in the same manner the corresponding attractive power of the earth hath a "considerable influence upon every one of their mo"tions also. The second supposition is this: that "all bodies whatsoever, that are put into direct and simple motion, will so continue to move forwards "in a straight line, till they are by some other effectual powers deflected and bent into a motion describing a circle, ellipsis, or some other more compounded curve line. The third supposition is, that these attractive powers are so much the more powerful in operating, by how much nearer the body wrought upon is to their own centres. Now what "these several degrees are I have not yet experi"mentally verified." But these degrees and proportions of the power of attraction in the celestial bodies and motions were communicated to Mr. Newton by R. Hooke in the year 1678 by letters, as will plainly appear both by the copies of the said letters, and the letters of Mr. Newton in answer to them, which are both in the custody of the said R. H., both which also were read before the Royal Society at their public meeting, as appears by the Journal Book of the said Society. "But it is a notion which, if 66 66 66 66 y In the Journal Book of the Royal Society there is no mention of any such correspondence in 1678, or in 1679 till Decem ber. It was a mistake therefore in Hooke to refer to the former of these two years. 66 66 "fully prosecuted as it ought to be, will mightily "assist the astronomer to reduce all the celestial mo"tions to a certain rule, which I doubt will never be "done true without it. He that understands the na"ture of the circular pendulum and circular motion, "will easily understand the whole ground of this principle, and will know where to find direction in "nature for the true stating thereof. This I only hint at present, to such as have ability and opportunity of prosecuting this inquiry, and are not wanting of industry for observing and calculating, wishing heartily such may be found, having myself many other things in hand, which I will first complete, and therefore cannot so well attend it. But "this I durst promise the undertaker. That he will find all the great motions of the world to be in"fluenced by this principle, and that the true understanding thereof will be the true perfection of astronomy." 66 66 66 66 66 66 66 Mr. Wood! This is the greatest discovery in nature, that ever was since the world's creation: it never was so much as hinted by any man before. I know you will do him right. I hope you may read his hand: I wish he had writ plainer, and afforded a little more paper. Tuus, J. AUBREY. Before I leave this town I will get of him a catalogue of what he hath wrote, and as much of his inventions as I can; but they are many hundreds; he believes not fewer than a thousand. "Tis such a hard matter to get people to do themselves right. |