SEQUEL TO THE EPOCH OF NEWTON. 245 



The effect of perturbations upon a comet defeats 

 all known methods of approximation, and requires 

 immense labour. " Clairaut," says Bailly 36 , " under- 

 took this : with courage enough to dare the adven- 

 ture, he had talent enough to obtain a memorable 

 victory:" the difficulties, the labours, grew upon 

 him as he advanced, but he fought his way through 

 them, assisted by Lalande, and by a female calcu- 

 lator, 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 errour, was made in haste, that 

 it might appear as a prediction. The comet justi- 

 fied 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 

 eccentric, 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 gravita- 

 tion. 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 

 intermediate case. 



We may add to the history of comets, that of 

 36 Bailly, A M. iii. 190. 



