PHYSICAL PROPERTIES. 23 



If, however, any appreciable amount of aqueous vapour or 

 suspended solid matter be present, both forms of radiant heat 

 are to a large extent absorbed. The presence of clouds con- 

 fers a still greater retentive power for heat. This effect of 

 aqueous vapour or of clouds is often very apparent at night ; 

 it is a matter of common experience that clear starlight or 

 moonlight nights, even in summer, are often cold, because of 

 the free radiation of heat from the earth into space, while 

 cloudy nights are generally much warmer. Water in the air, 

 too, has an important effect in conveying heat from one place 

 to another. Whenever water becomes gaseous, heat is ab- 

 sorbed, and when the vapour condenses again (often in the 

 upper regions of the air) heat is evolved. 



Air itself is thus little affected by the direct heat of the sun, 

 being heated either by contact with the hot surface of the 

 ground or by the aid of its own aqueous vapour. 



The specific heat of air is about -24, that of water being 

 unity, i.e., to raise the temperature of a given weight of air 

 through a given interval of temperature requires only about 

 one-fourth as much heat as would raise the temperature of the 

 same weight (or about ^^ of the volume) of water through the 

 same interval cf temperature. It is thus evident that by cool- 

 ing a given volume of warm water through one degree (or any 

 fixed interval of temperature) enough heat is extracted to raise 

 the temperature of about 3200 volumes of air by the same amount. 



Hence the potency of currents of warm water, e.y., the Gulf 

 Stream, in affecting climate and the slowness with which water 

 is cooled and converted into ice by cold winds, &c. 



The mean temperature of the atmosphere varies greatly, one 

 important factor being the latitude, which mainly determines 

 the amount of heat received from the sun. The temperature is 

 also found to vary greatly along the same parallel of latitude 

 with the nature of the soil and particularly with the proximity 

 or otherwise of large areas of water, places near the sea-coast 

 always enjoying a more uniform climate than those far inland. 

 Then, too, the height above the sea-level greatly affects the 

 temperature of a place, there being on the average a fall of 

 '1 C. for about every 400ft. above the sea-level but the rate is 

 very variable. 



