This is demonstrated by simple, yet decisive experiments. The discovery of the _fact_ is attributed to the illustrious Galileo, but to modern science we owe all the certainty, variety, and elegance of the demonstration. A vessel containing a quantity of air is weighed; the air is exhausted from it and it is weighed again. An accurate scale will then detect the difference of weight. A cubic foot of air weighs 1.2 oz. Hence a column of air of one inch in diameter and a mile in height weighs 44 oz. 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°. On the high plains of Quito, 8,724 feet above the sea, it boils at 194°, and an egg cannot be cooked there in an open vessel. At Potosí the boiling-point is still lower, being 188°, and the barometrical column stands at 18°. 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.