COMETS. 101 



lor 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 Ught with 

 greater intensity. Even in Herschel's large telescope, only 

 two comets, that discovered in Sicily in 1807, and the splen- 

 did one of 1811, exhibited well-defined disks ;* the one at an 

 angle of 1", and the other at 0""77, whence the true diame- 

 ters are assumed to be 536 and 428 miles. The diameters 

 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 nebu- 

 lous envelope were entirely separated from the tail by a darker 

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

 not augment toward the center in any uniform degree, bright- 

 ly shining zones being in many cases separated by concentric 

 nebulous envelopes. The tails sometimes appear single, some- 

 times, although more rarely, double ; and in the comets of- 

 1807 and 1843 the branches were of difierent leno^ths ; in 

 one instance (1744) the tail had six branches, the whole 

 forming an angle of 60^. The tails have been sometimes 

 straight, sometimes curved, either toward both sides, or to- 

 ward the side appearing to us as the exterior (as in 1811), or 

 convex toward the direction in which the comet is moving 

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

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

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

 through its center ; 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 century. These emanations 

 may be regarded as conoidal envelopes of greater or less thick- 



Maiue), 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 a{)peared like a very pure 

 white cloud, a darker space intervemng between the tail and the nu- 

 cleus. {Amer- Journ. of Science, vol. xlv., No. 1, p. 229.) 



* 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 <inJ 428 En 

 glish miles. For the magnitudes of the comets of 1798 and ISOJ. st;e 

 Arago, Annnaire, 1832, p. 203. 



a [The translator was at New Bedford, Massachusetts, U. S., on the 2Sth Februa- 

 ry, 1843, and distinctly saw the comet, between 1 and 2 in the afternoon. The skj 

 at the time was intensely blue, and the sun shining with a dazzling brightnoos un- 

 known in European climates.] — Tr 



