46 Mr. Galloway's TraJislation ofDr, Olhers^s 



of the distance from the sun nicreases,is still extremely doubtful. 

 A density assumed to diminish less rapidly than according to 

 this law, would have a great influence on the motion of a comet 

 which describes so much larger an orbit than Encke's. San- 

 tini found the acceleration of Biela's comet, computed on this 

 hypothesis, to be 0*03 of a day; whereas experience gave 0*45, if 

 not even 0*90 of a day. This likewise seems to make it probable 

 that the density of the aether diminishes more slowly, and ac- 

 cording to a different law ; perhaps the law of the ordinates 

 of a logarithmic curve. 3rd. It is not improbable that the 

 resisting medium is not at rest, but has a direct rotatory motion 

 about the sun. Even the perpetual revolution of the planets 

 must at length communicate to the aether through which they 

 move a direct motion of rotation ; but I am of opinion that such 

 a motion is coeval with the formation of our planetary system, 

 and was originally connected with it. Now, granting the di- 

 rect motion of the resisting medium, its effect on a comet 

 whose motion is retrograde, like Halley's, will be entirely 

 diff'erent from its effect on one whose motion is direct, like 

 Encke's. Experience alone can determine the amount of the 

 influence which the resistance of the aether has on the period 

 of revolution of Halley's comet. We have, it is true, the ex- 

 perience afforded by the return of the comet to its perihelion 

 in 1759; but in order to know precisely how much sooner the 

 comet arrived at its perihelion in 1759 through the effect 

 of resistance of the aether, it is indispensable to compute, 

 with the strictest accuracy, the amount of the perturbations 

 between 1607 and 1682. This would double the enormous 

 calculations which astronomers have undertaken for the pur- 

 pose of computing its return in 1835; a labour too arduous 

 to be expected of them, at least it has not yet been executed 

 by any one. 



Clairaut, when he undertook to calculate the return of 

 Halley's comet in 1759, did not content himself with computing 

 merely the perturbations from 1607 to 1682; he likewise 

 computed their amount for the revolution between 1531 and 

 1607. According to the results given by him in the Reckerches 

 surla Comete f Petersburg 1762), in which the first calculations 

 published in the Theorie des Cometes (Paris 1761) are revised 

 and in part corrected, the comet arrived at its perihelion, both 

 in 1682 and 1759, about 23 or 24 days before its computed 

 time. But no inference can be drawn from this as to the 

 actual amount of the influence of the resisting medium, in as 

 much as Clairaut was unacquainted with the existence of 

 Uranus, and assumed the mass of Saturn much too great, 

 namely = ^^^t- 



On account of this uncertainty as to the time of the perihe- 



