256 Comets. 



many more in ellipses. These latter, in so far as their orbits 

 can remain unaltered by the atti'action of the planets, must 

 be regarded as permanent members of our system. 



We must now say a few words on the actual dimensions 

 of comets. The calculation of the diameter of their heads, 

 and the lengths and breadths of their tails, offers not the 

 slightest difficulty when once the elements of their orbits are 

 known, for by these we know their real distances from the 

 earth at any time, and the true direction of the tail, which 

 we see only foreshortened. Now calculations instituted on 

 these principles lead to the surprising fact, that the comets 

 are by far the most voluminous bodies in our system. The 

 following are the dimensions of some of those which have 

 been made the subjects of such inquiry. 



The tail of the great comet of 1680, immediately after its 

 perihelion passage, was found by Newton to have been no 

 less than 20,000,000 of leagues in length, and to have occupied 

 only two days in its emission from the comet's body, a decisive 

 proof this of its being darted forth by some active force ; the 

 origin of which, to judge from the direction of the tail, must 

 be sought in the sun itself. Its greatest length amounted 

 to 41,000,000 leagues, a length much exceeding the whole 

 interval between the sun and the earth. The tail of the 

 comet of 1769 extended 16,000,000 leagues, and that of the 

 great comet of 1811, 36,000,000. The portion of the head of 

 this last, comprised within the transparent atmospheric en- 

 velope which separated it from the tail, was 180,000 leagues 

 in diameter. It is hardly conceivable, that matter once pro- 

 jected to such enormous distances, should ever be collected 

 again, by the feeble attraction of such a body as a comet — 

 a consideration which accounts for the surmised progressive 

 diminution of the tails of such as have been frequently ob- 

 served. 



We have b«3en somewhat diffuse on the subject of this 

 comet,* for the sake of shewing the degree and kind of in- 

 terest which attaches to cometic astronomy in the present 



* This refers to the Author's Account of the Great Comet of 1843, which we 

 have not, from want of space, extracted. — Edit. 



