The balloon is never entirely filled, for the atmospheric
pressure diminishing as it ascends, allows the hydrogen gas to
dilate, in virtue of its expansive force, and, unless there is
space for this expansion, the balloon is sure to explode in the
air.

An ordinary balloon, with a lifting power sufficient to carry up
three persons, with necessary ballast and materiel, is about
fifty feet high, thirty-five feet in diameter' and 2,250 cubic
feet in capacity. Of such a balloon, the accessories--the skin,
the network, the car--would weigh about 335 lbs.

To find out the height at which he has arrived, the aeronaut
consults his barometer. We know that it is the pressure of the
air upon the cup of the barometer that raises the mercury in the
tube. The heavier the air is, the higher is the barometer. At
the level of the sea the column of mercury stands at 32 inches;
at 3,250 feet--the air being at this elevation lighter--the
mercury stands at 28 inches; at 6,500 feet above sea level it
stands at 25 inches; at 10,000 feet it falls to 22 inches; at
20,000 feet to 15 inches. These, however, are merely the
theoretic results, and are subject to some slight variation,
according to locality, &c.

Sometimes the aeronaut makes his descent by means of the
parachute, a separate and distinct contrivance. If, from any
cause, it appears impracticable to effect a descent from the
balloon itself, the parachute may be of the greatest service to
the voyager at the present day it is chiefly used to astonish the