The atmosphere is supposed to have an elevation of from 45 to 50 miles, but its weight diminishes in proportion to its height.  The whole pressure at the surface of the earth is estimated to be 15 lbs. to the square inch; a person of ordinary size is consequently pressed upon by a weight of from 13 to 14 tons.  Happily for us, the pressure from without is counteracted by the pressure from within.

The weight of the air is of great importance in the economy of Nature, since it prevents the excessive evaporation of the waters upon the earth’s surface, and limits its extent by unalterable laws.  Water boils at a certain temperature when at the earth’s surface, where the weight of the atmosphere is greatest, but at different temperatures at different elevations from the surface.  At the level of the sea it boils at 212 deg..  On the high plains of Quito, 8,724 feet above the sea, it boils at 194 deg., and an egg cannot be cooked there in an open vessel.  At Potosi the boiling-point is still lower, being 188 deg., and the barometrical column stands at 18 deg..  Indeed, the experiment is often exhibited at our chemical lectures, of a flask containing a small quantity of water, which, exhausted of air, is made to boil by the ordinary heat of the hand.

Fahrenheit proposed to ascertain the height of mountains by this principle, and a simple apparatus was contrived for the purpose, which is now in successful use.  The late Professor Forbes of Edinburgh, whose untimely death the friends of science have had so much reason to deplore, ascertained that the temperature of boiling water varied arithmetically with the height, and at the rate of one degree of the thermometric scale for every 549.05 feet.  Multiplying the difference of the boiling-point by this number of feet, we have the elevation.  The weight of the atmosphere, as indicated by the barometer, is also a means for ascertaining the height of mountains or of plains; but correction must be made for the effects of expansion or contraction, and for capillarity, or the attraction between the mercury and the glass tube, at least whenever great exactness is required.  Tables for the convenience of calculation are given in several scientific works, and particularly in a paper of Professor Forbes, Ed. Trans.  Vol. 15.  Briefly, however, we may state, that between 0 deg. and 32 deg., 34 thousandths of an inch must be allowed for depression or contraction, and between 32 deg. and 52 deg. 33 thousandths.  The weight of the atmosphere is not only affected by rarefaction, but by currents of air, which give it a sudden density or rarity.  Those who have ascended mountains have experienced both these changes.