100 COSMOS. 



Althougli comets have a smaller mass than any other cos- 

 mical bodies — being, according to our present knowledge, prob- 

 ably not equal to joVo'^h part of the Earth's mass — yet they 

 occupy the largest space, as their tails in several instances ex- 

 tend over many millions of miles. The cone of luminous va 

 por 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 Sun, forming a line that intersects both the 

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

 vapor of the tails of comets mingled with our atmosphere in 

 the years 1819 and 1823. 



Comets exhibit such diversities of form, which appear rath 

 er to appertain to the individual than the class, that a de- 

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

 were already called by Xenophanes and Theon of Alexandria, 

 cotemporaries 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 vapor, with 

 the light concentrated toward the middle. This is the most 

 simple type ; but it can not, however, be regarded as rudi- 

 mentary, since it might equally be the type of an older cos 

 mical 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 characteristically de 

 nominated by the Chinese astronomers " the brush" {sui). 

 The nucleus generally presents no definite outline, 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, 



iug twenty-four hours within a distance of 2,000,000 miles from the 

 Earth) terrified Louis I. of France to that degree that he bUsied him 

 self in building churches and founding monastic establishments, in the 

 hope of appeasing the evils threatened by its appearance, the Chinese 

 astronomers made observations on the path of this cosmical body, whose 

 tail extended over a space of 60°, appearing sometimes single and' 

 sometimes multiple. The first comet that has been calculated solely 

 from European observations was that of 1456, known as Halley's coo- 

 et, from the belief long, but erroneously, entertained that the period 

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

 well-attested appearance. See Arago, in the Annuaire, 1836, p. 204, 

 and Laugier, Comptes Rendus des Stances de VAcad., 1843, t. xvi., 

 1006. 



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

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

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

 whose nucleus and tail were seen in North America on the 28th of Feb- 

 ruary (according to the tistimony of J. G. Clarke, of Portland, state of 



