WILLIAMSON ON COMETS. 193 



Having thus, as I think, made it perfectly clear that the former 

 theory of comets, which implies their excessive heat, cannot be correct, 

 I shall now try to show that comets are habitable globes. I stated above, 

 that the inhabitants of the several planets seem to be kept warm by the 

 weight of their respective atmospheres, which atmospheres seem to be 

 great or small, according to their greater or less distance from the sun ; 

 and the facts stated render it certain that we, on this globe, are indebted 

 to the atmosphere for the heat we enjoy. 



Now, according to our theory, as comets wander to a prodigious dis- 

 tance from the sun, (the comet of 1680 is supposed to have travelled to 

 the distance of twelve thousand millions of miles,) they cannot be kept 

 warm, in their aphelion, except by an immense covering. But it has 

 been computed that the atmosphere of that comet, which was as large as 

 the earth, is, at least, six hundred and fifty thousand times greater than 

 the atmosphere of the earth. This, as we presume, would be sufficient 

 for his winter coat. 



The chief difficulty that now presents itself, is to show by what 

 means a comet, pressed in its aphelion by an atmosphere some thou- 

 sands of miles high, can be so relieved, when it approaches the sun, as 

 not to be oppressed by heat. The manner in which the figure of that 

 vast body of air is changed, may be observed by every eye. As the 

 comet approaches the sun, his atmosphere is repelled by the sun, and 

 thrown into a long stream behind its body. That stream, or column of 

 air, which has been taken for a flame of fire, is nothing else than the 

 atmosphere of the comet, illuminated by the light of the sun. The light 

 is refracted by the outer part of the column, so that the whole becomes 

 visible, except a small part at the back of the nucleus. Having referred 

 to the repelling force of the rays of light, I am aware that I have 

 touched the most questionable part of this theory. It is perfectly clear 

 that the repulsion of the cometic atmosphere is from the body of the 



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