THE NEW SCIENCE OF METEOROLOGY 



from above in place of the heat which we radiate up- 

 ward. In towns, however, some compensation will be 

 afforded even on the clearest nights for the heat 

 which we lose in the open air by that which is radiated 

 to us from the sun round buildings. 



" To our loss of heat by radiation at times that we 

 derive little compensation from the radiation of other 

 bodies is probably to be attributed a great part of the 

 hurtful effects of the night air. Descartes says that 

 these are not owing to dew, as was the common opin- 

 ion of his contemporaries, but to the descent of certain 



; noxious vapors which have been exhaled from the earth 

 during the heat of the day, and are aftenvards con- 



I densed by the cold of a serene night. The effects in 

 question certainly cannot be occasioned by dew, since 

 that fluid does not form upon a healthy human body 

 in temperate climates ; but they may, notwithstanding, 

 arise from the same cause that produces dew on those 

 substances which do not, like the human body, possess 

 the power of generating heat for the supply of what 

 they lose by radiation or any other means." 2 



This explanation made it plain why dew forms on a 

 clear night, when there are no clouds to reflect the radi- 



it heat. Combined with Dalton's theory that vapor 

 is an independent gas, limited in quantity in any given 

 space by the temperature of that space, it solved the 

 problem of the formation of clouds, rain, snow, and 

 hoar-frost. Thus this paper of Wells's closed the epoch 

 of speculation regarding this field of meteorology, as 

 Hutton's paper of 1784 had opened it. The fact that 

 the volume containing Hutton's paper contained also 



191 



