The effects of a high temperature vary very much according to the amount
of moisture in the air, as when the air is nearly saturated in hot
climates, or even in summer in our own, more or less languor and malaise
are felt, with great indisposition to bodily labor. With a dry air these
are not so noticeable. The cause is evident; in the former case but little
evaporation occurs from the skin, and the normal amount of moisture is not
given off from the lungs, so that the body is not cooled down to such an
extent as by dry air. Sunstroke is probably the result, not only of the
direct action of the sun's rays, but partly from diminished cooling of the
blood by want of evaporation from the lungs and skin.
The effects of temperature on man do not depend so much on the mean for
the day, month, or year, as on the extremes, as, when the days are hot and
the nights comparatively cool, the energy of the system becomes partially
restored, so that a residence near the sea, or in the vicinity of high
mountains, in hot climates is, other things being equal, less enervating
than in the plains, as the night air is generally cooler. It is commonly
believed that hot climates are _necessarily_ injurious to Europeans, by
causing frequent liver derangements and diseases, dysentery, cholera, and
fevers.
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