202 ESSAY 



by the earth to the heavens, during the night, 

 when it receives from them little or no heat 

 in return. But, through the wise economy of 

 means, which is witnessed in all the operations 

 of Nature, the prevention of this evil is made 

 the source of great positive good. For the sur- 

 face of the earth, having thus become colder 

 than the neighbouring air, condenses a part of 

 the watery vapour of the atmosphere into dew, 

 the utility of which is too manifest to require 

 my speaking of it. I may remark, however, 

 that this fluid appears chiefly where it is most 

 wanted, on herbage, and low plants, avoiding, 

 in great measure, rocks, bare earth and con- 

 siderable masses of water*. Its production too, 



notwithstanding the immense quantities of heat that are ge- 

 nerated at its surface, by the continual action of the solar 

 rays?" Phil. Trans. 1804, p. 181. 



* I have no direct observations for the foundation of this 

 assertion concerning considerable masses of water. But, I 

 hold it, notwithstanding, to be just ; because, as soon as the 

 surface of the water is in the least cooled by radiation, the 

 particles composing it must fall downwards, from their in- 

 creased gravity, and be replaced by others that are warmer. 

 The whole mass, therefore, can never, in the course of a 

 single night, be sufficiently cooled to condense into dew any 

 great quantity of the watery vapour of the atmosphere. Be- 

 sides ; I have found, that even a small mass of water, as will 

 be more particularly mentioned in the last part of this Essay, 

 sometimes acquires no weight from the reception of dew, in 

 the space of a whole night favourable to the formation of 

 that fluid. 



