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 public, by showing them the spectacle of a man who, from a great elevation in the air, precipitates himself into space, not to escape dangers which threaten him in his balloon, but simply to exhibit his courage and skill. Nevertheless, parachutes are often of great actual use, and aeronauts frequently attach them to their balloons as a precautionary measure before setting out on an aerial excursion. The shape of a parachute, shown on the previous page, very much resembles that of the well-known all serviceable umbrella. The strips of silk of which it is formed are sewn together, and are bound at the top around a circular piece of wood. A number of cords, stretching away from this piece of wood, support the car in which the aeronaut is carried. At the summit is contrived an opening, which permits the air compressed by the rapidity of the descent to escape without causing damage to the parachute from the stress to which it is subjected.