SKCT. XXXVI. ENCKE'S AND BIELA'S COMETS. 349 



Every twenty-third year, or after seven revolutions of 

 Encke's cornet, its greatest proximity to Jupiter takes 

 place, and at that lime his attraction increases the pe- 

 riod of its revolution by nine days a circumstance 

 which took place in the end of the years 1820 and 1843. 

 But from the position of the bodies there is a diminution 

 of three days in the six following revolutions, which 

 reduces the increase to six days in seven revolutions. 

 Thus before the year 1819, the periodic time of Encke's 

 comei; was 1204 days, and it was 1219 days in accom- 

 plishing its last revolution, which terminated in 1845. 

 By this progressive increase the orbit of the comet will 

 reach that of Jupiter in seven or eight centuries, and 

 then by the very near approach of the two bodies it wiH 

 be completely changed. 



At present the earth and Mercury have the most 

 powerful influence on the motions of Encke's and Biela's 

 comets ; and have had for so long a time that, according 

 to the computation of Mr. Airy, the present orbit of the 

 latter was formed by the attraction of the earth, and 

 that of Encke's by the action of Mercury. With re- 

 gard to the latter comet, that event must have taken 

 place in February, 1776. Tn 1786 Encke's comet had 

 both a tail and a nucleus, now it has neither ; a singular 

 instance of the possibility of their disappearance. 



Comets in or near their perihelion move with pro- 

 digious velocity. That of 1680 appears to have gone 

 half round the sun in ten hours and a half, moving at 

 the rate of 880,000 miles an hour. If its enormous 

 centrifugal force had ceased when passing its perihe- 

 lion, it would have fallen to the sun in about three 

 minutes, as it was then less than 147,000 miles from his 

 surface. So near the sun, it would be exposed to a heat 

 27,500 times greater than that received by the earth ; 

 and as the sun's heat is supposed to be in proportion to 

 the intensity of his light, it is probable that a degree of 

 heat so intense would be sufficient to convert into vapor 

 every terrestrial substance with which we are acquainted. 

 At the perihelion distance the sun's diameter would be 

 seen from the comet under an angle of 73, so that the 

 sun, viewed from the comet, would nearly cover the 

 whole extent of the heavens from the horizon to tho 

 GG 



