Page:Wonderful Balloon Ascents, 1870.djvu/45

 it back by ropes; but as the force of ascension is every moment increasing, the work of restraining the balloon is most difficult and exciting. At length, all preparations being complete, the car is suspended, the aeronaut takes his seat, the words "Let go all!" are shouted, and away goes the silken globe into space.

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 matériel, 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