420

HISTORY OF PHYSICAL ASTRONOMY.

on the physical cause of the planetary movements, laboured under thedisadvantage of having only accessible to him the erroneous ideas of Kepler on the principles of mechanical science. A remark, however, contained inthe letter just cited, will serve to show' that he possessed a mind adequateto detect the fallacy of such ideas, and to substitute others more inaccordance with nature in their stead. Kepler had supposed the planetsto be whirled round in their orbits by the transverse action of magneticfibres; but as their revolutions round their axes seemed to him to offeran impediment to this action, he had recourse to the strange suppositionof the exterior stratum alone of each planet being endued with a rotatorymotion. Horrocks remarked that such a supposition was totally unneces-sary, since the rotatory motion of the planets could not impede theirmotion of translation, any more than the rotation of a stone thrown withthe hand impedes the motion which it has acquired in the direction of theimpulse. *

The following passages exhibit a distinct perception of the famousinequality in the mean motions of Jupiter and Saturn arising from theirmutual disturbance. It may be remarked that, throughout the whole ofthe sixteenth century and the first half of the seventeenth century,the mean motion of Jupiter was increasing with great rapidity in virtueof that inequality, while, on the other hand, the mean motion of Saturn was undergoing a corresponding retardation.

Writing on the 3rd of June, 1637, he mentions that he liad at length ob-tained possession of the Rudolphine Tables. A comparison of them withvarious modern observations convinced him that the mean motion ofJupiter was in reality quicker than Kepler had made it. On the 10th ofJanuary, 1638, he makes the same remark in nearly the same terms(ABqualis motus Jovis est notabiliter velocior qudm apud Keplerum). Onthe 25tli of July, 1638, lie proposes to correct the motion of Jupiter byadding 1° 30' to the aphelion, and about 2' to the mean longitude. In aletter dated September 29 of the same year, he states that the observationsof Jupiter made in the time of Waltherus agree with the most recentobservations in indicating, beyond all doubt, that the mean motion of theplanet is more rapid than it appeared to be from the Rudolphine Tables.The quantity of the acceleration seemed to him to amount to 1/ in tenyears. It was very certain, he added, that from the time of Tycho themean longitude of the planet was at least 4' or 5' greater than the tablesmade it. On the 14tli of September, 1639, he proposes to add 1' to themean longitude of Jupiter in the beginning of 1600, and 11' in the begin-ning of 1700, with a proportional quantity in any intermediate period. Inorder to show how nearly Horrocks arrived at the true acceleration, it maybe mentioned that the increments which the mean longitude of Jupiter ac-tually received in the first half of the seventeenth century, during successiveperiods of ten years, were 1' 22", 1' 20", V 18", l' 15", 1' 12". The timewhen Horrocks lived was, in fact, exceedingly favourable for detecting thegreat inequality of Jupiter and Saturn , on account of the rapidity withwhich it was then developing itself*. In a letter dated July 30, 1640,

* The trulh of this remark will be more apparent when it is stated that from 1800 to18.50 the accessions to the mean longitude of Jupiter , arising from the great inequalityduring successive periods of ten years, were only 10".8, 17".0, 22 '.9, 28".5, and84".1. These results, as well as those of a similar nature in the text, are calculated fromthe expression for the inequality given by Pontecoulant, (Theorie Analytique da Systcmedu Monde, tom. iii., p. 450), which includes the terms depending on the fifth powers of