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nitude and density, to that order, where the tail is first indicated, and then ascending higher in the scale, to that description whose vast projecting luminosities are thrown like arches across the sky. In general, however, comets are distinguished by a nucleus, which is brighter than the other parts, and probably of a density greater than the rest; an envelope surrounding the nucleus, and a tail, which is the continuation of the envelope in one direction, and that direction opposite to the sun. The component parts may be thus described: the nucleus the most brilliant part, and generally in the centre of the envelope, the envelope brighter than the tail, the tail which diminishes in luminosity as it recedes from the envelope, till lost in the surrounding ether.-/ See Plate 3.)
The Nucleus. We scarcely know how to place reliance on the observations of antiquity relative to the nuclei of comets; not merely owing to the imperfection of the instruments employed, but the dreaded influence of comets on the affairs of men, magnified the conceptions and deluded the judgments of observers. The most careful examination of the largest comet that has been observed within recent years (1811) scarcely furnished a measurable disc, namely, f of a second, and this quantity was supposed to be in excess; a luminous point was all that could be observed in one that appeared in 1813, which sparkled now and then, without the least appearance of a solid nucleus. In the second comet of 1825, (see plate 4,) the nucleus appeared as a milky irresolvable nebulosity, suddenly much brighter in the middle, but no outline as in a planetary disc; this brightest part was round, and from 10" to 15" in diame
ter, but in the centre not the least suspicion of a brilliant point, especially when compared with small stars near it, which exhibited sharply defined points. It is bence concluded, that the nucleus of a comet is merely nebulous matter, more or less consolidated; several comets that have been lately observed seemed merely collections of vapour, with a slight tendency to condensation towards their centres, and of such very rare and pellacid constitutions, that the smallest stars have been distinctly seen through their very centres; of this nature are the two comets which will return this year.
But though many of the accounts of comets we are compelled to receive with hesitation, there are others which deserve our attention from the high authority connected with their description. The nucleus of the comet of 1618 is said to have broken into three or four irregular figures resembling burning coals, which changed their situation during observation as when a fire is stirred: a few days after this, these pieces were broken into a greater number, and changed into a cluster of small stars. In the comet of 1661, the nucleus changed from a round form into several fragments; an appearance, of a similar description, was observed in the comet of Halley, when visible in the year 1607. Some comets have been observed with their nuclei of an oblong form, others divided into two by a black separation in the middle: such peculiarities as these have not been remarked in any comet seen with modern instruments, and by modern observers; it is true, that no comet has appeared of late years, which at all approximates to those which shook their tresses over the heavens in the years 1652, 1680, and 1682.