222 ESSAY 



foot, though it should absorb the same pro- 

 . portional quantity of what enters it. In this 

 way, every successive foot will acquire a less 

 quantity of heat than the preceding, and a 

 state of the atmosphere be produced, like to 

 that which is actually observed in a calm and 

 sunny day. In the day, however, the pheno- 

 mena, from the heating of air by rays from the 

 earth, are somewhat confused by the warmed 

 portions rising upwards, and mixing with what 

 is colder ; whereas, at night, the air, which has 

 been cooled by radiating heat to the earth, is 

 rendered, by an increase of gravity, the more 

 fit to retain its low position. I have here, for 

 the sake of simplifying the argument, taken no 

 notice of the cooling of any considerable mass 

 of the air, in consequence of the actual contact 

 of its lowermost stratum with the earth, or by 

 the conduction of the temperature of one por- 

 tion of it to another. But, in a calm state of 

 the atmosphere, these effects must be incon- 

 siderable, though it appears to me impossible, 

 in the present state of our knowledge, to de- 

 termine them with any precision. 



According to the view, which has been given 

 by me of this subject, the heat of the air, in a 

 clear and calm night, ought to increase, within 

 the limits of the phenomenon, in some de- 

 creasing geometrical ratio, as the atmosphere 



