348 THE SHOCK OP A COMET. SECT. XXXVI. 



At all events, there is no proof of this having occurred; 

 and it is manifest that the axis of the earth's rotation 

 has not been changed, because, as the ether offers no 

 sensible resistance to so dense a body as the earth, the 

 libration would to this day be evident in the variation it 

 must have occasioned in the terrestrial latitudes. Sup- 

 posing 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 retrogade mo- 

 tions 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 approach of a comet, with- 

 out 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 61 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 axi-s 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, 



