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HISTORY OF PHYSICAL ASTRONOMY.

of the moon or some other equivalent. It is clearly possible to establish'any principles whatever, if we are at liberty to have recourse to such :assumptions in support of our reasoning. It will he remarked that jKepler does not seek to explain how the motion of the moon in her orbit iis continually kept up; he doubtless assigned this task to the animal force jwhich regulated the distance between the two bodies. The difficulty of ■accounting for the motion of a body in its orbit, by means of a centripetralforce, occurs to him perpetually throughout the Astronomia Nova in courseof his speculations on the physical cause of the planetary motions. Inattempting to explain the phenomena of these motions by means of aforce emanating from the sun, he is now compelled, like Gilbert, to intro-duce a principle totally at variance with his previous notions of gravitation ;for he imagines that the planet requires to be continually impelled in itsorbit by the solar force. To meet this view of the case, he supposes thesun to revolve from west to east, upon an axis perpendicular to the planeof the ecliptic, and to send forth continually magnetic rays, which attractthe planet in a direction transverse to the line joining it and the sun.

It is hardly necessary to state that this opinion of the planets beingkept revolving by a force continually whirling them round in' their orbitsis not only at direct variance with the character of a gravitating force, butis also inconsistent with the fundamental principles of motion. It must beadmitted that there was more of truth in Ross’s w r ords than he could perhapsjustly take credit for, when he asserted that “ Kepler ’s opinion, that theplanets are moved round by the sunne, and that this is done by sendingforth a magnetic virtue, and that the sunbeames are like the teethe of awheele taking hold of the planets, are senselesse crotchets fitter for awheeler or a miller than a philosopher.” *

Kepler might have formed more accurate ideas on the physical cause ofthe planetary motions, if the science of mechanics had been more advancedin his time; but it is surprising that, although he constantly strovethroughout his researches on the planet Mars , as detailed by him in theAstronomia Nova , to connect the varying motion of the planet with aforce emanating from the sun, he nowhere speculates so judiciously onthat force as in the introduction to his work; and at the conclusion of hislabours he inspires no more confidence in his reader respecting the realityof the force than he did at the commencement of them. In fact, it is tothe extraordinary tenacity with which he clung to the idea of a solar forceacting somehow on the planets, and his strong conviction that theirmotions were regulated by fixed law's, that we must ascribe the brilliantresult of his researches, rather than to any clear perception either of thenature of the force or of its mode of operation.

It is difficult to say whether Gilbert or Kepler was first led to speculateon the physical theory of the celestial motions. Kepler ’s earliest notionson the subject are to be found in his “ Mysterium Cosmographicum, ” whichwas published in 1596. Gilbert’s “ Treatise on the Magnet ” appeared in1600, and he died in 1603, leaving behind him his posthumous work,which was published only in 1651. It is clear from the nature ofGilbert’s ideas, which turn entirely upon the magnet, that they could nothave been suggested to him by Kepler ’s speculations. It is equallycertain that the latter was not indebted to any person for his opinion {

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* The New Planet no Planet, or the Earth no Wandering Star, 4to., London , 1C46.See also Life of Kepler_L. U. K.