. In this
way the process of evaporation is carried on very rapidly and the heat
is extracted from the water to such an extent that it freezes, often
forming ice in one night over an inch in thickness, and this in the
hottest climates on the globe. Evaporation cannot go on in places where
the air is already saturated with moisture. When the air is dry
evaporation is very rapid, but as it becomes more and more filled with
moisture the evaporation is checked to the same degree. This fact
accounts for the difference of bodily comfort that we experience at
different times in the year when the temperature is the same. Sometimes
we are very uncomfortable although the temperature is not above 75
degrees Fahrenheit, more so even than we are at other times when the
temperature is ten or fifteen degrees higher. If the air is saturated
with moisture, even though the temperature is not above 70 or 75
degrees, the perspiration is not readily evaporated from the surface of
the body. If the air is dry the temperature may be much higher and we be
much more comfortable, because evaporation goes on rapidly, which keeps
the body not only dry, but cool. I remember passing through a desert in
Arizona where there was scarcely a green thing in sight in any
direction, and the temperature was said to be 140 degrees. I did not
suffer as much as I often have done in the East with the thermometer at
80 or 90 degrees, and there was very little show of sensible
perspiration; it was going on rapidly, however, but was being absorbed
by the dry air. This goes to show that temperature is not the only
factor to be considered when we are making an estimate of the good or
bad qualities of a climate.

Evaporation is carried on much more rapidly when the wind blows than at
other times, for the reason that the moisture is carried off laterally
as fast as it is formed, all resistance to its escape into the upper air
being removed. If the air is charged to saturation with moisture at a
certain temperature, it will remain so, and evaporation stops so long
as the temperature remains unchanged. If its temperature rises the
process of evaporation can start up, because the capacity of the air for
holding moisture has been increased. But if a temperature is perceptibly
lowered another phenomenon will manifest itself.

In the uncondensed state vaporized moisture is quite transparent, so
that we are able to see through it as we do through a pane of glass. If,
however