368 POSSIBILITY OF COLLISION. SECT. XXXV. 



envelop a portion of the earth's orbit, a circumstance which 

 caused some alarm in France, from the notion that, if any dis- 

 turbing cause had delayed the arrival of the comet for one month, 

 the earth must have passed through its head. M. Arago dis- 

 pelled these fears by his excellent treatise on comets, in the 

 Annuaire of 1832, where he proves that, as the earth would 

 never be nearer the comet than 18,000,000 British leagues, there 

 could be no danger of collision. The earth is in more danger 

 from these two small comets than from any other. Encke's 

 crosses the terrestrial orbit sixty times in a century, and may 

 ultimately come into collision, but both are so extremely rare, 

 that little injury is to be apprehended. 



The earth would fall to the sun in 64^ days, if it were struck 

 by a comet with sufficient impetus to destroy its centrifugal 

 force. What the earth's primitive velocity may have been it is 

 impossible to say. Therefore a comet may have given it a shock 

 without changing the axis of rotation, but only destroying part 

 of its tangential velocity, so as to diminish the size of the orbit 

 a thing by no means impossible, though highly improbable. At 

 all events, there is nor 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. 

 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 re- 

 stored, 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, without being 

 touched by it. The comets that have come nearest to the earth 



