132 
J. HOPKINSON-WATER AND WATER-SUPPLY 
Water evaporates at all temperatures; even the coldest ice gives 
off some vapour; but it is only when the water is boiling that the 
vapour is formed below the surface. Transparent aqueous vapour 
is 37^ per cent, lighter than atmospheric air, each particle of water, 
on conversion into vapour, absorbing heat enough to raise the 
temperature of 960 such particles one degree. The amount of 
evaporation increases as the temperature rises, as the air is drier, 
and as the surface exposed to the air becomes greater. From still 
water there is, therefore, the least amount of evaporation; when 
the water is agitated, the evaporation increases ; from rough moist 
ground it is greater than from water; and from ground covered 
with vegetation it is at least doubled. Wind, also, increases the 
amount of evaporation by passing dry or comparatively dry air 
rapidly over the moist surface, which thus more freely parts with 
its moisture than it would do if that moisture remained longer in 
contact with it. 
The circulation of water in the atmosphere, brought about by the 
vapour falling as rain (or snow), and thus allowing the sun to draw 
up more vapour to take the place of that which is abstracted, is 
only in part due to the fact that warm air can retain more moisture 
than cold air, so that a cold current meeting and mixing with 
a warm moist current supersaturates the air, causing some of the 
moisture it contains to fall as rain. If the temperature and 
humidity varied in the same proportion, all mixtures of currents of 
different temperature would have the mean degree of humidity of 
the two currents ; but the amount of vapour air can contain not only 
increases with the temperature, but it increases in a greater ratio 
than does the temperature. It follows from this that the air formed 
by the mixture of any two saturated currents of air of different 
temperature will always be supersaturated. 
The amount of evaporation forms a measure of the heating power 
of the sun, and as that is greatest in the tropics and decreases 
towards the poles, the evaporation must vary correspondingly. 
But the rainfall also is greatest in the tropics, and it is doubtful how 
much of the heat drawn up with vapour from tropical seas is con¬ 
veyed to higher latitudes and there liberated when rain falls. Still 
there is a constant circulation going on in the atmosphere as there 
is in the ocean, by which warmth is conveyed from the hotter to 
the colder parts of the earth, thus ameliorating the climate, cold 
currents flowing in the reverse direction. Into the details of this 
atmospheric and oceanic circulation it is unnecessary to enter here ; 
biit it may be of interest to give one instance of the heat due to 
rainfall. On the west coast of Ireland the mean rainfall is about 
45 inches, and the heat given out by this amount of rain is nearly 
half that due to the direct rays of the sun. It has been calculated 
by Professor Haughton^ that while the direct heat of the sun re¬ 
ceived there in one year would melt a layer of ice 61-4 feet thick, 
the indirect heat of the sun due to rainfall would in the same time 
melt 30 feet of ice. 
* ‘ Lectures on Physical Geography,’ p. 128. 
