SECT. XXXV. POSSIBILITY OF COLLISION. 369 



were that of the year 837, which remained four days within less 

 than 1,240,000 leagues from our orbit : that of 1770, which 

 approached within about six times the distance of the moon. 

 The celebrated comet of 1680 also came very near to us ; and 

 the comet whose period is 6f years was ten times nearer the earth 

 in 1805 than in 1832, when it caused so much alarm. 



Encke's and Biela's comets are at present far removed from 

 the influence of Jupiter, but they will not always remain so, 

 because, the aphelia and nodes of the orbits of these two 

 comets being the points which approach nearest to the orbit 

 of Jupiter at each meeting of the planet and comets, the 

 major axis of Encke's comet will be increased and that of 

 Biela's diminished, till in the course of time, when the proxi- 

 mity has increased sufficiently, the orbits will be completely 

 changed, as that of Lexel's was in 1770. Every twenty-third 

 year, or after seven revolutions of Encke's comet, its greatest 

 proximity to Jupiter takes place, and at that time his attraction 

 increases the period of its revolution by nine days a circum- 

 stance 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 comet was 1204 days, and it was 

 1219 days in accomplishing the revolution that ended 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 will 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 regard to the latter comet, that event must 

 have taken place in February 1776. In 1786 Encke's comet 

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

 instance of the possibility of their disappearance. It was in 

 perihelio in 1855. 



In 1846 Biela's comet was divided into two distinct bodies, 

 by what strange accident is altogether a mystery. The nuclei 



B 3 



