COMETS. 85 



Although comets have a smaller mass than any other cos- 

 mical bodies being, according to our present knowledge, 

 probably not equal to 50 ! 00 part of the Earth's mass yet they 

 occupy 'the largest space, as their tails in several instances 

 extend over many millions of miles. The cone of luminous 

 vapour which radiates from them has been found, in some 

 cases (as in 1680 and 1811), to equal the length of the Earth's 

 distance from the Sim, forming a line that intersects both the 

 orbits of Venus and Mercury. It is even probable that the 

 vapour of the tails of comets mingled with our atmosphere in 

 the years 1819 and 1823. 



Comets exhibit such diversities of form, which appear rather 

 to appertain to the individual than the class, that a description 

 of one of these " wandering light-clouds," as they were 

 already called by Xenophanes and Theon of Alexandria, 

 contemporaries of Pappus, can only be applied with caution 

 to another. The faintest telescopic comets are generally 

 devoid of visible tails, and resemble Herschel's nebulous stars. 

 They appear like circular nebulae of faintly- glimmering 

 vapour, with the light concentrated towards the middle. 

 This is the most simple type ; but it cannot, however, be 

 regarded as rudimentary, since it might equally be the type 

 of an older cosmica? body, exhausted by exhalation. In the 

 larger comets we may distinguish both the so-called " head " 

 or " nucleus," and the single or multiple tail, which is charac- 

 teristically denominated by the Chinese astronomers " the 

 brush." (sui.) The nucleus generally presents no definite out- 

 line, although, in a few rare cases, it appears like a star of the 

 first or second magnitude, and has even been seen in bright 

 sunshine;* as, for instance, in the large comets of 1402, 1532, 

 1577, 1744, and 1843. This latter circumstance indicates, in 

 particular individuals, a denser mass, capable of reflecting 

 light with greater intensity. Even in Herschel's large tele- 

 it was first observed by that astronomer was its first and only well 

 attested appearance, See Arago, in the Annvaire, 1836, p. 204, and 

 Laugier, Comptes rendus ties tieanc.es de I'Acad., 1843, t. xvi. 1006. 



* Arago, Annuaire, 1832. pp. 209, 211. The phenomenon of the tail 

 of a comet being visible in bright sunshine, which is recorded of the 

 comet ot 1402, occurred again in the case of the large comet of 1843, 

 whose nucleus and tail were seen in North Air.erica, on the 28th 

 of February (according to the testimony of J. G. Clarke, of Portland, 



