SECT, xxxvr.] SHOCK OF A COMET. 397 



as the earth, the libration would to this day be evident in 

 the variation it must have occasioned in the terrestrial lati- 

 tudes. Supposing the nucleus of a comet to have a diameter 

 only equal to the fourth part of that of the earth, and that 

 its perihelion is nearer to the sun than we are ourselves, its 

 orbit being otherwise unknown, M. Arago has computed 

 that the probability of the earth receiving a shock from it 

 is only one in 281 millions, and that the chance of our 

 coming in contact with its nebulosity is about ten or 

 twelve times greater. Only comets with retrograde motions 

 can come into direct collision with the earth, and if the 

 momentum were great the event might be fatal ; but in 

 general the substance of comets is so rare, that it is likely 

 they would not do much harm if they were to impinge ; and 

 even then the mischief would probably be local, and the 

 equilibrium soon restored, provided the nucleus were gaseous, 

 or very small. It is, however, more probable that the earth 

 would only be deflected a little from its course by the ap- 

 proach of a comet, without being touched by it. The comets 

 that have come nearest to the earth 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 cele- 

 brated comet of 1680 also came very near to us; and the 

 comet whose period , is 6| 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 which 

 shall take place there, the major axis of Encke's comet will be 

 increased, and that of Biela's diminished, till in the course of 

 time, when the proximity has increased sufficiently, the orbits 

 will be completely changed, as that of Lexel's was in 1770. 



