86 COSMOS. 



scope only two comets, that discovered in Sicily, in 1807, and 

 the splendid one of 1811, exhibited well-defined discs;* the 

 one at an angle of I", and the other at 0"'77, whence the true 

 diameters are assumed to be 536 and 428 miles. The dia- 

 meters of the less well defined nuclei of the comets of 1798 

 and 1805 did not appear to exceed 24 or 28 miles, 



In several comets that have been investigated with great 

 care, especially in the above-named one of 1811, which con- 

 tinued visible for so long a period, the nucleus and its nebulous 

 envelope were entirely separated from the tail by a darker 

 space. The intensity of light in the nucleus of comets does 

 not augment towards the centre in any uniform degree ; 

 brightly shining zones being in many cases separated by 

 concentric nebulous envelopes. The tails sometimes appear 

 single, sometimes, although more rarely, double ; and in the 

 comets of 1807 and 1843 the branches were of different 

 lengths ; in one instance (1744) the tail had six branches, the 

 whole forming an angle of 60. The tails have been some- 

 times straight, sometimes curved, either towards both sides, 

 or towards the side appearing to us as the exterior (as in 

 1811), or convex towards the direction in which the comet is 

 moving (as in that of 1618) ; and sometimes the tail has even 

 appeared like a flame in motion. The tails are always turned 

 away from the sun, so that their line of prolongation passes 

 through its centre ; a fact which, according to Edward Biot, 

 was noticed by the Chinese astronomers as early as 837, but 

 was first generally made known in Europe by Fracastoro and 

 Peter Apian, in the sixteenth centuiy. These emanations may 

 be regarded as conoidal envelopes of greater or less thickness, 



State of Maine), between 1 and 3 o'clock in the afternoon.* The distance 

 of the very dense nucleus from the sun's light admitted of being 

 measured with much exactness. The nucleus and tail appeared like a 

 very pure white cloud, a darker space intervening between the tail and 

 the nucleus. (Amer. Journ. of Science, vol. xlv. No. 1., p. 229.) 



a [The translator was at New Bedford, Massachusetts, U.S., on the 28th 

 February, 1843, and distinctly saw the comet, between 1 and 2 in the 

 afternoon. The sky at the time was intensely blue, and the sun shining 

 with a dazzling brightness unknown in European climates.] Tr. 



* Phil Trans, for 1808, Part II. p. 155, and for 1812, Part I. p. 118. 

 The diameters found by Herschel for the nuclei were 538 and 428 

 English miles. For the magnitudes of the comets of 1798 and 1805 

 see -Arugo, Annuaire, 1832, p. 203. 



