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The atmosphere possesses, as may be readily imagined, many important
characteristics. One of these is Weight.

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