Robert Hooke on Gravity

“Having enumerated some of the most remarkable Proprieties of Gravity, we come in the next place to consider what may be the Cause thereof. And first, I believe I shall not need to say much against the Opinion of Intelligent Matter, which supposes every part of Matter to act understandingly, for that being supposed, all Philosophy is vain, and there needs no farther Inquiry into Nature. And secondly, I have as little to say to its Cousin-german Opinion, viz. the Regimen of an Hylarchick Spirit. And 3ly, The Epicurean Atoms seem to me to give as little of Explanation almost as either of the former. And 4ly, For the Peripatetick Doctrine of tendency to the Center of the Universe, besides that the Foundation is false, the Earth being proved not to be in the Center, 'tis not yet understood what the tendency is. 5ly, The Cartesian Doctrine and that of Mr. Hobbs are both insufficient, because they do not give any reason why Bodies should descend towards the Center under or near the Poles. 6ly, Nor will the Magnetism of Gilbert or Kepler serve; for, as I shall afterwards shew, that is a Propriety distinct from Gravity, and of quite another nature.”

“I could... largely explain, and plainly evince, that the Motions of several Bodies at a distance, are caused by the internal motion of the founding Body; and that this Power of moving is every way propagated by the ambient Medium, which excites in solid Bodies at a distance, a similar Motion. I could farther also prove, that every one of these distinct internal Motions of Bodies, as that of Light, and that of Sound, have distinct and differing Mediums, by which those Motions are communicated from the affecting to the affected Body: And so I conceive also that the Medium of Gravity may be distinct and differing both from that of Light, and from that of Sound. I conceive then, that the Gravity of the Earth may be caused by some internal Motion of the internal or central Parts of the Earth; which internal and central Motion may be caused, generated and maintained by the Motion of the external and all the intermediate parts of its Body: So that the whole Globe of the Earth may contribute to this Motion, as it will happen to a Globe of Glass or solid Mettal, to any part of which no internal Motion can be communicated, without at the same time affecting the whole with the same Motion. And I shall most plainly and evidently prove, when I come to the Explication of Magnetism, that this is undeniably performed and effected by this means.”

“'I should here have described some Clocks and Time-keepers of great use, nay absolute necessity in these and many other Astronomical observations, but that I reserve them for some attempts that are hereafter to follow, about the various wayes I have tryed, not without good success of improving Clocks and Watches and adapting them for various uses, as for accurating Astronomy, completing the Tables of the fixt stars to Seconds, discovery of Longitude, regulating Navigation and Geography, detecting the properties and effects of motions for promoting secret and swift conveyance and correspondence, and many other considerable scrutinies of nature: And shall only for the present hint that I have in some of my foregoing observations discovered some new Motions even in the Earth it self, which perhaps were not dreamt of before, which I shall hereafter more at large describe, when further tryalls have more fully confirmed and compleated these beginnings. At which time also I shall explaine a Systeme of the World, differing in many particulars from any yet known, answering in all things to the common Rules of Mechanicall Motions: This depends upon three Suppositions. First, that all Cœlestial Bodies whatsoever, have an attraction or gravitating power towards their own Centers, whereby they attract not only their own parts, and keep them, from flying from them, as we may observe the Earth to do, but that they do also attract all the other Cœlestial 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 motion of the Earth, and the Earth upon them, but that Mercury also, Venus, Mars, Saturne, 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 motions also. The second supposition is this, That all bodys whatsoever that are put into direct and simple motion, will so continue to move forward in a streight 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 Centers. Now what these several degrees are I have not yet experimentally verified;'—But these degrees and proportions of the power of attraction in the celestiall bodys and motions, were communicated to Mr. Newton by R. Hooke in the yeare 1678, by letters, as will plainely appear both by the coppys 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 Royall Society at their publique meeting, as appears by the Journall book of the said Society.—'but it is a notion which if fully prosecuted as it ought to be, will mightily assist the astronomer to reduce all the Cœlestiall motions to a certaine rule, which I doubt will never be done true without it. He that understands the natures 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 my self many other things in hand which I would first compleat, 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 influenced by this Principle, and that the true understanding thereof will be the true perfection of Astronomy.'”