156 NOMOS. 



met with in the focus, is up to the very limits of 

 amazement. 



Now it is an argument in favour of this explana- 

 tion, that the nucleus and tail are dependent upon the 

 same cause for their existence, and that this cause is 

 one which operates equally upon the earth. Indeed, 

 it is possible to fancy that the earth itself might be 

 furnished with a tail if the point at which the sun's 

 rays were made to converge was moved so near to 

 the surface as to act powerfully upon that surface 

 and upon the atmosphere above it. Nay, it is pos- 

 sible to fancy that the earth does actually present the 

 appearance of a small tail over the crater of a volcano 

 in active operation, for the molten lava must occasion 

 a tremendous upward expansion of the superjacent 

 atmosphere. We think, then, that the possibility 

 of accounting for the presence and changes of the 

 nucleus, and for the formation of the tail, by suppos- 

 ing that the comet is composed of distinct concentric 

 layers with wide intervening spaces, is an argument 

 of some force in favour of the actual existence of 

 these layers and spaces. But this is not the only 

 argument, as we shall see presently. 



The diminution in size which the comet undergoes 

 as it approaches the sun, has been ac- 

 counted for by supposing that a part of 

 * ts substance has been vaporised by the 

 may be ac- increasing heat, and this, no doubt, is the 



counted for > ' ' 



in the same proper explanation. And for the same 



manner. . . _ .. 



reason it is possible to understand that 

 the tail should also decrease as the comet approaches 

 the sun. There is no difficulty in this. But there 



