THE FORMATION OF DEW. 58 



having a damper atmosphere. The cause of this difference 

 is, however, not to be sought for in any greater clearness of 

 the atmosphere in dry than in damp climates, as the fact is 

 rather the reverse of this, the air being somewhat clearer in 

 the damp countries; but in a process which, when dew is 

 formed, always counteracts to a certain extent the cooling 

 effect of radiation. 



When radiation cools the surface of the earth so much as 

 to condense and liquefy some of the aqueous vapour that is 

 in the air, that liquefaction liberates much heat: and this 

 heat tends lo warm the part that is in course of being cooled 

 by radiation. There is then a double process going on at 

 the same time and in the same place! Radiation is cooling 

 the part, whilst liquefaction of vapour is warming it; and, un- 

 der these circumstances, it is only to the extent that the in- 

 jfiuence of the former exceeds the latter, that cooling is ac- 

 complished. When there is much vapour in the atmosphere, 

 much of it is soon liquefied, and the cooling effect of radiation 

 is thereby counteracted to a great extent; when there is little 

 vapour, there is less of liquefaction of that vapour, and cool- 

 ing is consequently less counteracted. And where the 

 atmosphere is so dry as not to admit the liquefaction of any 

 vapour from the degree of cold that exists, radiation produces 

 its effect without being in any degree counteracted by 

 recently liberated heat 



In such dry deserts as those referred to, the cold in the 

 early part of the night, when it produces dew, produces 

 it only on the best radiators, which are generally the few 

 vegetables that are found in the deserts; and, as the cold 

 increases, worse radiators have dew deposited on them suc- 

 cessively in the order of their radiating powers. 



In our own country, from the operation of the cause 

 here pointed out, radiation does not produce that intense 

 cold in the early part of the winter, when the dew-point is 

 comparatively high, that it does at a later period of the 



