'So far I have explained the phenomena of our heavens and the sea by the force of gravity : but I have not yet assigned the cause of gravity. Certainly this force arises from some cause, which penetrates even to the centre of the sun and planets, without diminution of its virtue ; and which acts not aecerding to the superficial magnitude of particles (as do many mechanical causes), but according to the quantity of solid matter; and of which the action is extended in every direction to immense distances, diminishing always in the duplicate ratio of the distances. Gravity towards the sun is compounded of the gravity towards all the several particles of the sun, and in receding from the aim diminishes accurately in the duplicate ratio of the distances, as far as the orbit of Saturn, as is made manifest by the quiescence of the aphelia of the planets, and even as far as the aphelia of the comets, if they also do not move. But I have not yet been able to deduce the reason of these properties of gravity from phenomena, and 1 do not frame hypotheses. For whatever is not deduced from phenomena, is to be called hypothesis: and hypotheses, whether metaphysical or physical, or of occult qualities, or mechanical, have no place in r.rperimental philosophy. In this philosophy propositions are deduced from phenomena, and are rendered general by induction. So impene trability, mobility, the impetus of bodies, and the laws of motion anti gravity have become known. And it is enough that gravity really exists and acts according to laws explained by me, and suffices for all the motions of the heavenly bodies and of our sea.
" I might add something on that most subtilo spirit which pervades solid bodies and lies hidden in them, by the force and action of which the particles of bodies attract each other at the smallest distances, and adhere when contiguous : and by which electric bodies act at greater distances, both by attracting and repelling neighbouring particles ; and by which light is emitted, reflected, refracted, inflected, and gives heat to bodies; and by which all sensation is excited, and the limbs of animals are moved at pleasure, namely, by its vibrations through the solid capillaments of the nerves, propagated from the external organs of sense to the brain, and from the brain to the muscles. But this could not be explained in few words ; nor is there a sufficient quantity of experiments to show and determine the laws according to which this spiritus acts." There is not a sentence of the preceding scholium which has not v direct reference to some common opinion, at home or abroad, of the physics, metaphysics, or theology of Newton's system. lie had probably seen and heard enough, in the interval between the editions to teach him on what points explanation was desirable ; certain it is that there is hardly an objector since his time who might not have found himself already answered, if he had read this scholium wit] candour and attention.
The commentators of Newton have one and all, as might be supposed, treated their author as a source of elementary instruction to be explained and cleared from the effects of that brevity ani obscurity (the latter more than due to the former) which the Principle almost everywhere presents. But the time has arrived when till, work is hardly in the beginner's course, though it may he recommendec to him, when he has studied the modern analysis, to make himscl acquainted with its methods until he has caught their spirit. Wha we now want is rather an historical commentary, which shall put thi student in possession of the modes of reasoning peculiar to Newton', predecessors, shall point out how the Principle came to have its form snd shall place him, so far as is possible, in the midst of that Mine 'There of remnants of the old philosophy and aspirations after the new in which the mind of Newton gained its growth.
In 1730, Dr. John Clarke published A Demonstration of some o the principal sections of Sir I. Newton's Principles,' kc. This wort contains the greater part of the first book somewhat expanded, ant with the applications of the third book intermixed. But it omits fir most important part of the eleventh section : nevertheless, a studen who should join with Dr. Clarke's work the article GRAVITATION ii he present one, would have the most important parts of the Principle, o far as is necessary to gain an insight into Newton's methods.
In the same year, 1730, George l'eter Domck published his' Philoe. lath. Newt. Illustrates Toni Duo, Londini.' (This work is sometimes vrongly called an edition of the Principia, and its author's name is oinetimes spelled Donick.) The first volume of it is only a prepa atory course of mathematics; the second gives a largo part of the first aaok in Newton's manner, and gives more of the results of the eleventh section. It also enters upon some of the numerical applications of the bird book.
The Commentary of the 'Minims (Jesuits they are usually but wrongly called) he Sucur and Jacquicr (1739) is an excellent perform ince for its time considered as attempting to smooth the details of the . .
mathematical It uses algebra freely, but is totally insufficient to show the use of the differential calculus as now known ; but it very frequently developer satisfactorily a point at which Newton only hinted.
Emerson's 'Short Comment on Newton's Principle,' 1770, is a brief explanation of some of the mathematical difficulties and obscurities, Followed by defences of the Prineipia, the Optics, and the Chronology. Emerson defended everything of Newton's.