188 WILLIAMSON ON COMETS. 



bited by rational beings, and little more exposed to excessive heat or 

 cold than we are. 



As objections have been published to some allegations contained in 

 that theory, I have reviewed the subject with some attention, since the 

 appearance of a late comet in the year 1811; but no objections have 

 come under my notice by which the conclusions I formerly drew are, 

 in any degree, impaired. True it is, that it has been questioned whe- 

 ther the rays of the sun's light have any force by which they might 

 propel the particles of air. It has also been objected that the theory 

 of heat, which I formerly mentioned, was not correct. The present 

 theory of heat is confessedly very different from that which I formerly 

 adopted ; nor shall I pause a minute to consider whether this or that 

 be most correct ; since it is not necessary, because it fortunately hap- 

 pens, that the conclusions I attempted to draw do not require the sup- 

 port of a doubtful theory. This I shall endeavour to show by a diffe- 

 rent course of reasoning. 



In order to form correct ideas concerning planets to which we have 

 no access, we are bound to suppose that they agree, in certain promi- 

 nent characters, with the only planet with which we are well acquainted. 

 From our knowledge, then, of the globe we inhabit, we venture to con- 

 clude; 



1 . That an atmosphere, or air, is essential to animal or vegetable life. 



2. That the heat which prevails on the surface of any planet is 

 according to the weight of its atmosphere, and not inversely as the 

 .square of its distance from the sun. 



3. That all planets, or other bodies revolving round the sun, are 

 provided with an atmosphere that is great in proportion to their several 

 distances from the sun. 



4. That the luminous tail of a comet is nothing else than part of its 

 atmosphere thrown to a considerable distance behind the comet, for the 

 relief of the cometarians in their approach towards the sun. 



