HISTOBY OF PHYSICAL ASTRONOMY.
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he still adheres to his previous conclusion. lie proposes to add 5' to themean longitude at the beginning of 1040, and O' at the beginning of1050. Throughout the whole of the current century, the mean motion ofthe planet seemed to him to be more rapid than it was according to theKudolphiue Tables, the acceleration being equivalent to an increase in themean longitude of 1' in ten years. “ Whether the acceleration will con-tinue or not,” says he, “ I do not know; but between 1400 and 1590 themean motion was sensibly more rapid than it is now.” He here seems tohint at the possible periodicity of the phenomenon.
The retardation of Saturn ’s mean motion did not escape the sagacityof Horroeks, although he does not seem to have retained such a firm holdof the inequality as in the case of the corresponding acceleration ofJupiter ’s mean motion. On the 3rd of June, 1037, he proposes tosubtract 4' from the mean longitude of Saturn at the beginning of 1000.On the 15tli of October, 1038, he writes that the observations of Saturn ,in the time of Waltherus, indicated the mean motion of the planet tobe slower than Kepler had made it in the ltudolphine Tables. On the14th of September, 1039, he again alludes to the irregularity in the meanmotion of the planet. “ Saturn, ” he says, “ seems to experience sometimes asingular retardation in its motion. ( Videtur Saturnus miram aliquamaliquando habere motus sui retardationem.) He remarks, however, thatthe phenomenon would occasion him greater annoyance were it not thatthere was some consolation in being probably the first who discoveredit (nisi quod hoc aliqwid solatii est, nos, credo, primos esse qui detegirnus'j.Ho requests Crabtree to watch the phenomenon carefully by making con-stant observations of the planet. With respect to the true correctionto be applied to the Kudolphiue Tables he was unable to pronounce apositive opinion ; “ but if we discover anything hereafter,” says he, “ theretardation of the planet will allow an easy correction of the Ephemerides .”
In a letter dated July 26, 1638, he makes some conjectures relativeto the nature and movements of comets. He supposes them to be pro-jected from the sun, and to move with a continually slower velocity asthey recede from that body, until at length they become stationary, andthen begin to return with accelerated speed, like the sine in the circle,deflecting somewhat in the direction of the sun’s rotation. He requestsCrabtree to send him Tycho Brahe ’s observations of comets, containedin his Progymnasmata, in order that he might compare them with hishypothesis. In a letter dated September 29, 1638, he states that theobservations of the comets of 1577 and 1590 confirmed his conjecture ofcomets in general being projected from the sun. He supposes, in accord-ance with Kepler’s ideas of a whirling force, that they are subsequentlycarried round the sun with a motion which is, in all probability, variable*.
On the 3rd of October, 1640, he announced to Crabtree that he hadundertaken the prosecution of a series of observations on the tides, inorder that, by the aid of experiment, he might be enabled to arrive atsome definite conclusions respecting their real nature.
the eccentricities and inclinations of the two planets, and also those due to the square ofthe disturbing force.
* Wallis, in a letter to the Royal Society dated January 21, 1664-5, states that Horroeksmade the comet to return to the sun in an elliptical figure, or very nearly so. Accordingto this hypothesis he traced the comet of 1577. He requests that the paper relative tothis comet, being in Horroeks’ own handwriting, be carefully preserved (Birch, Hist.Roy. Soc., vol. ii., p. 11).