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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.