36 EQUILIBRIUM OF TEMPERATURE. 



account for this phenomenon we must remember that the 

 temperature of the thermometer is stationary, only so long as 

 it receives as much heat as it radiates. It is in that state 

 before the experiment is made with the ice ; for the air or any 

 object which may happen to be in the other focus is of the same 

 temperature as the ball of the thermometer. But it is evident 

 that the moment ice is introduced into one focus, less heat will 

 be sent from that to the other focus, than was previously trans- 

 mitted^ and than is necessary to sustain the thermometer at 

 a constant temperature. The thermometer ball, therefore,, 

 giving out as much heat as formerly and receiving less in return, 

 must fall in temperature. This is an experiment in which the 

 thermometer ball is in fact, the hot body. 



The doctrine of the radiation of heat was very happily applied 

 to account for the deposition of Dew. A considerable refrigera- 

 tion of the surface of the ground below the temperature of the 

 air resting upon it, amounting to 10 or 20 degrees, occurs 

 every calm and clear night, and is caused by the radiation of 

 heat from the earth (which is a good radiator) into empty space. 

 Now on becoming colder than the air above it, the ground will 

 condense the moisture of the air in contact with it, and be 

 covered with dew. For the air, however clear, is never 

 destitute of watery vapour, and the quantity of vapour which 

 air can retain depends upon its temperature, air at 32, for in- 

 stance, being capable of retaining l-150th of its volume of vapour 

 w r hile at 52' it can retain so much as l-S6th of its volume. The 

 greatest difference between the temperature of the day and night 

 in this country takes place in spring and autumn, and these are 

 the seasons in which the most abundant dews are deposited. 



That the deposition of dew depends entirely upon radiation is 

 fully established by the following circumstances ; 1. It is on 

 clear and calm nights only that dew is observed to fall. When 

 the sky is overcast with clouds, no dew falls ; for then the heat 

 which radiates from the earth is returned by the clouds above, 

 and prevented from escaping into space ; so that the ground 

 never becomes colder than the air. 2. The slightest screen, 

 such as a thin cambric handkerchief, stretched between pins, at 

 the height of several inches above the ground, is sufficient to 

 protect the objects below it from this chilling effect of radiation, 

 and to prevent the formation of dew or of hoar-frost upon 

 them. This fact was well known to gardeners, and they had long 



