TERRESTRIAL HEAT. 



rain or mist taking the liquid form before its actual deposition on 

 the surface. 



The deposition of dew, however, also takes place even where the 

 atmosphere is not reduced to its point of saturation. "When the 

 firmament is unclouded after , sunset, all objects which are good 

 radiators of heat, among which the foliage and flowers of vegetables 

 are the foremost, lose by radiation the heat which they had 

 received before sunset without receiving any heat from the firma- 

 ment sufficient to replace it. The temperature of such objects, 

 therefore, falls much below that of the air, on which they produce 

 an effect precisely similar to that which a glass of very cold water 

 produces when exposed to a warm atmosphere charged with 

 vapour. The air contiguous to their surface being reduced to the 

 dew point by contact with them, a part of the vapour which it 

 holds in suspension is condensed, and collects upon them in the 

 form of dew. 



It follows from this reasoning, that the dew produced by the 

 fall of temperature of the air below the point of saturation will be 

 deposited equally and indifferently on the surfaces of all objects 

 exposed in the open air, but that which is produced by the loss of 

 temperature of objects which radiate freely, will only be deposited 

 on those surfaces which are good radiators. Foreign writers on 

 physics accordingly class these depositions as different phenomena, 

 the former being called by French meteorologists serein, and the 

 latter rosee or dew. We are not aware that there is in English 

 any term corresponding to serein. 



Dew will fail to be deposited even on objects which are good 

 radiators, when the firmament is clouded. For although heat be 

 radiated as abundantly from objects on the surface of the earth as 

 when the sky is unclouded, yet the clouds being also good 

 radiators, transmit heat, which being absorbed by the bodies on 

 the earth, compensates for the heat they lose by radiation, and 

 prevents their temperature from falling so much below that of the 

 air as to produce the condensation of vapour in contact with 

 them. 



Wind also prevents the deposition of dew by carrying off the 

 air from contact with the surface of the cold object before con- 

 densation has time to take place. Meanwhile by the contact of 

 succeeding portions of air, the radiator recovers its temperature. 



In general, therefore, the conditions necessary to insure the 

 deposition of dew is, 1st, a warm day to charge the air with 

 vapour ; 2nd, an unclouded night ; 3rd, a calm atmosphere ; and, 

 4th, objects exposed to it which are good radiators of heat. 



In the close and sheltered streets of cities the deposition of 

 dew is rarely observed, because there the objects are necessarily 

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