HISTORY OF PHYSICAL ASTRONOMY.

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men who, from Hipparchus downwards, by their successive efforts, esta-blished the fundamental facts relative to the movements of the planetswithout reference to their physical cause.

Although the posthumous works of Horrocks can only be regarded asmere fragments, their perusal cannot fail to excite a feeling of deepregret that astronomy was so soon deprived of the benefit of his labours.It would he inconsistent with the object of this work to attempt a com-plete analysis of their contents, but a glance over some of his letters toCrabtree may serve to confirm the truth of Delambre’s remark, that if theirauthor had lived he would have proved himself the worthy successor ofKepler. These letters were written originally in English , but weretranslated into Latin by Dr. Wallis.

In a letter dated November 23, 1037, Horrocks states that he had re-cently spent some time in meditating upon the physical principle in virtuoof which the planets revolve in oval orbits. “ Kepler,” says he, “ attri-butes their movements to the action of magnetic fibres, but I entertainserious objections to this hypothesis. It appears to me, however, that Ihave fallen upon the true theory, and that it admits of being illustratedby means of natural movements on the surface of the earth, for natureeverywhere acts according to a uniform plan, and the harmony of creationis such that small things constitute a faithful type of greater things."

In a letter dated July 25, 1638, he ascribes the motion of the lunarapsides to the disturbing force of the sun. This very remarkable idea of aperturbative influence, exercised by the various bodies of the planetary sys-tem upon each other, had not yet been suggested by any philosopher. Thecircumstance of its being perfectly true in the particular case alluded toby Horrocks, tends in a still greater degree to enhance the merit of thesurmise. In the same letter he exhibits an illustration of the planetarymovements by suspending a weight from a fixed point by a long cord, and,having drawn the weight a little aside from the vertical direction, apply-ing to it a slight tangential impulse. This beautiful experiment, illustra-tive of the action of a central force, has been generally ascribed to Hooke,who merely reproduced it, at a meeting of the Koyal Society , about thirtyyears after Horrocks had devised it*. Horrocks remarked that the pen-dulous body would describe an ellipse, the apsides of which would advanceslowly in the direction of the body’s motion. It did not escape his obser-vation that this experiment did not represent the movements of theplanets with sufficient fidelity, inasmuch as there were two perihelia andtwo aphelia, the centre of the ellipse being in fact the centre of force,instead of the focus, as in the case of nature. In order to make theparallel more complete, he supposed a gentle wind to blow constantlyupon the pendulum in the direction of the major axis of the ellipse. Ifthe representation was still imperfect, it must be admitted that the stephere taken was at least in the right direction. Horrocks, in his researches

* Hooke first gave an account of his experiment at the meeting of the Royal Society ,which was held on the 23rd of May, 1666 (Birch’s “ History of the Royal Society,” vol. ii.,p. 92). It might be supposed that as Horrocks’ posthumous works were not published till1672, Hooke could not have been indebted to them for the original idea of the experiment.It appears, however, that Wallis had completed his task of preparing the writings of Hor-rocks for publication as early as September 21, 1664 (Birch, vol. i., p. 470). The print-ing of the work was deferred by the Council until the president should give his opinionrespecting it. The circumstance of its remaining so long in manuscript arose, doubtless,from the impoverished state of the Society ’s treasury during the early period of itsexistence.