Iconographic Encyclopedia (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1860 edition. Excerpt: ...The tails also become broader towards their extremity, and are often divided longitudinally in their middle by a dark line, so that it seems as if the tail were double. This was plainly perceived on the 10th of Sept. 1811 in the great comet, as shown in fig. 16. The sun undoubtedly produces the tail, as this is always first visible when the comet approaches the sun, becomes larger as the approximation increases, and again diminishes with the gradual increase of distance between the two bodies. Consequently the tails appear, for the most part, to consist of very thin vapor developed by the heat of the sun from the nucleus of the comet. The alterations arising from this cause, which, according to numerous observations, must often be enormous, and may take place even within a few days, doubtless produce the changes observed in the size, shape, and brilliancy of comets. When a comet becomes visible to the naked eye, it is generally seen but a short time, and has a very different course in.the heavens from the planets, though it follows the usual daily motion of the heavens. Formerly, on account of the rarity of comets, their remarkable appearance, and their course, it was supposed that they were not true heavenly bodies. Newton, however, first showed that they, like the planets, are heavenly bodies belonging to our solar system. Like the planets, they describe, according to the same laws, orbits around the sun. The eccentricity of their orbits is very great. It. is now also known that telescopic comets, or those only visible to the eye when assisted by a good telescope, occur in far greater numbers than others, and that multitudes of them are probably always present in the space belonging to our solar system. There are thus far, only three...