For OUT Water Supply, 



249 



radiation, because, as soon as the surface particles lose any 

 portion of heat, their density is at the same time increased, 

 and they sink to a lower level, being replaced by warmer 

 particles from underneath. 



Thus water differs most materially from grass and other 

 vegetable bodies whose power of radiation is very great, and 

 which therefore cool very rapidly, and being very bad con- 

 ductors, the heat that is lost by radiation is very slowly 

 restored from the ground. Hence in clear calm nights they 

 condense dew in great abundance. 



The greater the depth of water, the more slowly is its 

 temperature diminished, as the surface cannot lose even 1^ 

 of heat until the whole depth has been reduced to the same 

 temperature. And in this dry climate the dew point or point 

 of saturation is often many degrees below the temperature of 

 the air. It is thus easy to see that when there is a depth of 

 more than a few inches of water no dew can be condensed on 

 its surface. 



But we are not left to determine this point by reasoning 

 on general principles. It is fortunately one that can very 

 readily be determined by experiment. 



Dr. Wells found that a thermometer laid on a grass plot in 

 a clear night, and in calm weather, sunk G'^, S'^, 13^, and 

 even 20^ lower than a thermometer hung at some height 

 from the ground. This explains the rapid extraction of heat 

 from the atmosphere in contact with the grass plot, and the 

 copious deposition of dew on grass. But no such rapid re- 

 duction of temperature has ever been observed in water placed 

 under similar circumstances. The surface of the ocean and 

 inland lakes retains a very uniform temperature, corresponding 

 to the seasons, and suffers little change from the ordinary 

 alternations of heat and cold during day and night; indeed 

 the difference in the temperature of the ocean is scarcely 

 perceptible. 



In temperate regions, the difference in the diurnal range 

 of the thermometer in the air over the ocean is very trifling, 

 rarely exceeding from 4^ to 6*^, while upon the continents 

 the range often amounts to 20"^ or 30*^, and between the 

 latitudes of 25"^ and 50^ the air is rarely warmer than the 

 surface of the sea. And it is found by careful observation 

 that while the temperature of the air over the land is rapidly 

 cooled by the chilling influence of radiation during the night, 

 the air over the ocean is several degrees colder than the surface 

 of the water, and is therefore heated, not chilled, by contact 

 with its warmer surface. 



