HISTOBY OF PHYSICAL ASTBONOMY.

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relative to the existence of some physical principle directing the motionsof the planets. When, however, he attempted at a subsequent period of:his researches to devise a consistent theory of the solar force, he adopted■the views of Gilbert by assuming it to be a modified form of magnetism.This appears from his great work , the “ Astronomia Nova, ”* wherein hecites the opinion of G ilbert while proceeding to frame his theory of awhirling force.

Galileo , by means of his admirable researches on mechanics, contributedin a high degree towards the formation of more distinct ideas on thesubject of curvilinear motion. The principle of mutual gravitation doesnot seem, however, to have found any favour with him, for he censures■ Kepler on account of his opinion relative to the attraction of the earth bythe moon. He admitted the attraction of the moon by the earth, but heby no means formed a distinct conception of the mode in which the forceof gravity in this case operates. “ The parts of the earth,” f says he,“ have such a propensity to its centre, that when it changes its place,although they may be very distant from the globe at the time of thechange, yet must they follow. An example similar to this is the perpetualsequence of the Medicean stars, although always separated from Jupiter.The same may be said of the moon obliged to follow the earth.”

The earth’s attraction is here evidently inferred from the moon con-stantly attending her in her annual orbit round the sun. It might, how-ever, be concluded from the same phenomenon, with equal shew ofreason, that the moon attracts the earth; for the moon cannot be said tofollow the earth any more than the earth can be said to follow the moon,since, in fact, both bodies, while revolving round the sun, revolve alsocontinually round their common centre of gravity. The grand fact whichleads to the establishment of the action of the earth upon the moon,consists in the revolution of the latter in a curvilinear orbit which isconcave with respect to the earth. It has been sometimes said that Kepleronly required a more complete knowledge of the laws of motion in orderto have demonstrated the existence of the principle of gravitation. Here,however, we have a philosopher equal in sagacity to Kepler—who hadsuccessfully analyzed the phenomenon of curvilinear motion in one of itsmanifestations at least, and who moreover had access to the opinions ofKepler on the subject of gravitation; still, notwithstanding all these ad-vantages, he failed to recognise the existence of an attractive force, eitherin the motion of the moon round the earth, or in the motions of theplanets round the sun. This circumstance ought to render us cautious inattaching an undue value to mere sagacious surmises unsupported bylegitimate proof, and in ascribing to individuals any credit for discoverieswhich are not the actual result of their own labours.

We do not propose to make any further allusion to Descartes ’ theory ofvortices, beyond the few words we have already said respecting it in theintroduction to this work. No doubt, we think, can exist that thiscelebrated fiction exercised a most pernicious influence in retarding theprogress of sound mechanical ideas relative to celestial physics. Likethe theory of solid orbs, it at length utterly disappeared before the ad-vancing light of true science, after continuing for nearly a century to

* Astronomia Nova , cap . xxxv., p. 176.

+ Dialago sopra i due Massimi Sistemi del Mondo.—Firenze, 16.32. See alsoLife of Galileo_L. U.K.

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