WILLIAMSON ON COMETS. J 8t» 



1. Knowing that air is an essential article in every business of life, 

 we naturally associate the idea of an atmosphere to every planet in 

 which we presume there may be animal or vegetable life. Without 

 the benefit of air, fire cannot burn, vapours cannot ascend; there can 

 neither be rain nor dew; there cannot be any speech, for sounds can- 

 not be propagated ; life could not be preserved. 



2. If heat came in direct rays from the sun, it would follow, that the 

 quantity of heat on the surface of every planet would be inversely as 

 the square of its distance from the sun, according to a former opinion. 

 But experiments without number prove that such opinion is unfounded. 

 Although light comes from the sun, and light has the power of exciting 

 heat, it is clear that heat does not come from the sun. The experiment 

 is conclusive in all warm climates, and in every climate where there are 

 high mountains. At the foot of the Andes, near the equator, the 

 inhabitants suffer by the heat of eighty or ninety degrees. Let them 

 ascend the mountain, and before they have risen two miles, they find 

 themselves in the region of perpetual snow. How should this happen? 

 They are surely nearer the sun on the mountain than upon the plain, 

 and they are subjected to more of his rays ; therefore, according to the 

 alleged rule, they should be warmer; but they are, in fact, much 

 colder, because they are in a thinner, or a lighter, atmosphere. The 

 issue is the same in all cases where the experiment has been made. On 

 the Peak of TenerifFe, the traveller arrives at the region of frost at the 

 very same height, or under the same weight of the atmosphere, that he 

 finds it upon the Andes. If he could rise but two miles farther, it is not 

 to be questioned that he would, in so thin an atmosphere, perish by the 

 want of heat. Hence we infer that the degree of heat, in every planet, 

 is according to the weight of its atmosphere compared with its distance 

 from the sun. We also infer that the inhabitants of the several planets 



