110 Esmark on the Geological History of the Earth. 



There are several phenomena by which these last are distin- 

 guished from the planets. They revolve like them round the 

 sun, but in much more ecGentn<^ orbits. The period of the re- 

 volution of a comet is very different from one century to ano- 

 ther.* Their greatest distance frofia the sun is so immense, that 

 if men could exist upon them, they would not see the sun for 

 thousands of years, and the degree of cold must be such, that if 

 there were sea or water upon them, it must be in a state of ice. 

 When these bodies are advancing to their perihelium, and at 

 different distances approach nearer the earth, we observe that 

 they are not only surrounded by a luminous atmosphere, but 

 that they have likewise a long luminous tail, brth of which be- 

 come the greater the nearer they come to the sun, decreasing 

 in the same manner as they remove from him to a greater dis- 

 tance. With regard to this increase and decrease of light too, 

 we oliserve a difference among them . In the case of some of 

 them, almost the whole mass of the comet is changed into this 

 luminous elastic atmosphere and tail ; in others we perceive a 

 distinct red nucleus, which, on its approach to the sun, has a less 

 expanded atmosphere and tail. These atmospheres, and still 

 more the tails, are so thin and elastic, that, without the least ob- 

 struction, we can see through them the lesser stars. Counsellor 

 Huth has calculated, that the luminous matter in the tail of the 

 great comet of 1811, was a million of times rarer than our at- 

 mosphere at the surface of the sea. The volume of its tail he 

 computed to be 2000 times the bulk of the sun, and the diame- 

 ter of its nucleus to be eighteen times that of the earth. Bessel 

 calculates the period of its revolution to be 3383 years. Her- 

 schel has accurately observed this comet, and, among other things, 

 he concludes, that its light was peculiar to itself. The colour 

 of its nucleus was greenish, or a bluish green, and the nucleus 

 or head was not in the centre of the atmosphere, which was most 

 expanded on that side turned to the sun. The radius of the 

 atmosphere he makes to be about 322,000 English miles, and 

 the length of the tail more than 100 millions. By continued 

 observations on this comet, he found that it underwent actual 

 physical changes in its structure, and that it was globular. On 



* Their number is considered as exceeding 4000. 



