Esmark on the Geological History of the Earth. Ill 



its approach to the sun, his light and heat seemed to produce 

 chemical effects upon it ; he believed that it revolved on its axis. 

 By comparing this comet with that of 1807, he concludes, that 

 every time comets approach their perihelium, they come nearer 

 and nearer to the sun ; that, therefore, the comet of 1807 had 

 several times been in its perihelium, it beng 25 million of times 

 nearer the sun than the comet of 1811. Its tail was only 9 mil- 

 lions of miles long, whereas that of 1811 was 91 millions. The 

 effect of the sun on the latter was much greater than on the 

 other, and it had probably seldom or never before been in its 

 perihelium ; whereas, on the contrary, the comet of 1807, in 

 consequence of having been several times in its perihelium, must 

 be more advanced in its growth, and matured. By comparing 

 likewise the constitution of the great comet, with a smaller one 

 in the same year 1811, he found that the smaller one was much 

 more complete, and approached nearer the nature of a planet 

 than the great comet, as the influence of the sun upon its peri- 

 helium, was not much greater than his influence on a planet in 

 the same situation *. 



I have made these remarks on the nature of comets, and this 

 comparison of some of them, for the sake of introducing certain 

 considerations, drawn from facts, which I consider as a proof 

 that our earth, in its rude and undigested state, has been a co- 

 met. Of this hypothesis, I shall state, in what follows, several 

 strong proofs in phenomena which occur in our own country. 



Whiston, in his Theory of the Earth, supposes that it has 

 originally been a comet, or the atmosphere of a comet, which, 

 in its course round the sun, has moved in a very eccentric el- 

 hpse; that, of course, in its perihelium, it has been subjected to 

 a very high degree of heat, and in its aphelium, to an ^equally 

 strong degree of cold ; that, in the one of these situations, it 

 was vitrified by the heat, in the other covered with ice ; that, 

 by degrees, its orbit has gradually changed from this long el- 

 lipse, to that almost circular path in which it now moves on the 

 whole, at a much smaller distance from the sun. How far does 

 this agree with the phenomena which we can observe on our 



• See Phil. Transact. 1811, and Boede*s Astronomisches Jahrbuch fur das 

 Yahr 181G, p. 185. 



