The weight of the atmosphere at the level of the sea is equal to about
fourteen and one-half pounds on every square inch of surface. This
weight is balanced by a column of mercury thirty inches in height; but
at an elevation of 18,000 feet it would be balanced by a column of only
fifteen inches in height, and at 36,000 by one only seven and one-half
inches in height. It is on this principle that the mercurial barometer
has been constructed; and since the mercury in the barometer stands at
the same point at all places at the sea-level, and falls in a regular
ratio on ascending therefrom, this instrument forms a most useful
standard for measuring altitudes.
As we ascend from the sea the atmosphere becomes colder; but, as with
the density, the temperature does not appear to pass through regular
gradations of change. From experiment, however, it has been assumed that
the atmosphere loses one degree of heat by Fahrenheit’s thermometer for
every 350 feet of ascent; and hence even in the hotter regions very
lofty mountains are covered with perpetual ice and snow.