ON DEW, &c. 251 



what we radiate to the heavens. At night also, 

 if the sky be overcast, some compensation will 

 be made to us, both in towns and in the 

 country, though in a less degree than during 

 the day, as the clouds will remit towards the 

 earth no inconsiderable quantity of heat. But 

 on a clear night, in an open part of the country, 

 nothing almost can be returned to us from 

 above, in place of the heat which we radiate 

 upwards. In towns, however, some compensa- 

 tion 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 by the sur- 

 rounding buildings. 



To our loss of heat by radiation, at times 

 that we derive little compensation from the ra- 

 diation of other bodies, is probably to be at- 

 tributed a great part of the hurtful effects of 

 the night air. Descartes * says that these are 

 not owing to dew, as was the common opinion 

 of his cotemporaries, but to the descent of cer- 

 tain noxious vapours, which having been ex- 

 haled from the earth during the heat of the 

 day, are afterwards condensed by the cold of a 

 serene night. The effects in question certainly 

 cannot be occasioned by dew, since that fluid 



* Meteorolog. c. vi. 



