22 
most solids. The formation of ice is thus a most powerful 
means of arresting a decline of temperature, as evaporation 
is the great natural remedy for excessive heat. The heat 
required to melt 20 inches depth of frozen water over the 
whole surface of any portion of land is equal to that of 37 hours 
of vertical sunshine, if we adopt the datum of Sir J. Herschel, 
that vertical solar heat on a square foot in one second would 
raise one pound about one-ninth of a degree. The total 
summer heat, reckoned roughly, would be equal to 1,300 hours 
of vertical heat at the equator, or lat. 54°, and 900 hours at 
the pole : hence, if the whole winter rainfall were deposited 
in snow or ice, the heat needed to melt the whole would be 
that of four days only nearest to the summer solstice, or one- 
thirtieth of the whole summer heat in our latitude. 
38. The reasoning in “ Climate and Time,” pp. 58, 59, seems 
to assume that ice and snow are the cause and not the effect 
of a cold climate, and tend to aggravate not to mitigate its 
severity. But the exact opposite is true. As ocean currents 
tend to equalize the temperatui-e of different parts of the 
earth, so the formation and melting or evaporation of ice and 
snow are the chief natural means of lessening the difference 
of sensible heat in different seasons of the year. When the 
radiation is in excess of the supply of solar heat, the freezing 
of water sets free 140° of heat to repair the loss; and 
when the summer returns, all the ice and snow must be 
melted before the temperature can have a sensible rise abovo 
the freezing-point. A pound of water, with a sensible differ- 
ence of 180° only from its frozen state to its evaporation at 
the boiling-point requires 1,320° of heat, and this will be 
equivalent to 5,280° or 6,600° degrees for a pound of rock or of 
earth, the specific heat being one-fourth or one-fifth of that of 
water. Or, taking the interval from zei’O to 70°, a pound of water, 
in virtue of the process of freezing and its great specific heat, 
serves to reduce the sensible change of temperature from 
twelve to fifteen times. 
39. There are three ways in which snow and ice are said to 
lower the summer temperatures. First by direct radiation. 
Whatever the heat of the sun, the snow and ice can never rise 
above 32°, and their radiation lowers all surrounding bodies 
to that level. Next, the rays which fall on them are to a great 
extent reflected into space, and those which are not reflected, 
but absorbed, disappear in the mechanical work of melting 
tlio ice. Thirdly, they chill the air, and condense tho moisture 
into fogs, and these provont tho sun's rays from reaching tho 
earth ; thus the snow, in theso aphelion winters, would remain 
unmcltcd the whole summer. 
