For our 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. y 
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 6°, 8°, 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. 
Tn 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. 
FF 
