SEQUEL TO THE EPOCH OF NEWTON. 451 



But in all this, the Comet had been supposed to be affected only by 

 the attraction of the sun. The planets must disturb its motion as they 

 disturb each other. How would this disturbance affect the time and 

 circumstances of its reappearance ? Halley had proposed, but not 

 attempted to solve, this question. 



The effect of perturbations upon a comet defeats all known methods 

 of approximation, and requires immense labor. " Clairaut," says 

 Bailly, 38 " undertook this : with courage enough to dare the adventure, 

 he had talent enough to obtain a memorable victory ;" the difficulties, 

 the labors, grew upon him as he advanced, but he fought his way 

 through them, assisted by Lalande, and by a female calculator, Madame 

 Lepaute. He predicted that the comet would reach its perihelion 

 April 13, 1759, but claimed the license of a month for the inevitable 

 inaccuracy of a calculation which, in addition to all other sources of 

 error, was made in haste, that it might appear as a prediction. The 

 comet justified his calculations and his caution together ; for it arrived 

 at its perihelion on the 13th of March. 



Two other Comets, of much shorter period, have been detected of 

 late years ; Encke's, which revolves round the sun in three years and 

 one-third, and Biela's, which describes an ellipse, not extremely eccen- 

 tric, in six years and three-quarters. These bodies, apparently thin and 

 vaporous masses, like other comets, have, since their orbits were calcu- 

 lated, punctually conformed to the law of gravitation. If it were still 

 doubtful whether the more conspicuous comets do so, these bodies 

 would tend to prove the fact, by showing it to be true in an interme- 

 diate case. 



[2d Ed.] [A third Comet of short period was discovered by Faye, 

 at the Observatory of Paris, Nov. 22, 1843. It is included between 

 the orbits of Mars and Saturn, and its period is seven years and three- 

 tenths. 



This is commonly called Faye's Comet, as the two mentioned in the 

 text are called Encke's and BieMs. In the former edition I had ex- 

 pressed my assent to the rule proposed by M. Arago, that the latter 

 ought to be called Gambarts Comet, in honor of the astronomer who 

 first proved it to revolve round the Sun. But astronomers in general 

 have used the former name, considering that the discovery and obser- 

 vation of the object are more distinct and conspicuous merits than a 

 calculation founded upon the observations of others. And in reality, 



38 Bailly, A. M. iii. 190. 



