. The Family tutor . ifically lighter than theair, through which, of course, it will rise. (Fig. 44.) The hydrogen gas balloon consists, in like manner, of athin, impervious bag, filled either with hydrogen or commoncoal gas. The former, as usually made, is from ten to thirteentimes lighter than air; the latter is somewhat heavier. A balloonfilled with either of these possesses, therefore, a great ascensional power, and will riseto considerable heights. Thus, Biot and Gay Lussac, in 1804, ascended in one of thesemachines to an elevation of 23,000 feet. When the balloon first ascends, it oughtn
. The Family tutor . ifically lighter than theair, through which, of course, it will rise. (Fig. 44.) The hydrogen gas balloon consists, in like manner, of athin, impervious bag, filled either with hydrogen or commoncoal gas. The former, as usually made, is from ten to thirteentimes lighter than air; the latter is somewhat heavier. A balloonfilled with either of these possesses, therefore, a great ascensional power, and will riseto considerable heights. Thus, Biot and Gay Lussac, in 1804, ascended in one of thesemachines to an elevation of 23,000 feet. When the balloon first ascends, it oughtnot to be full of gas, for as it reaches regions where the pressure is diminished, thegas within it is dilated, and though flaccid at first, it will become completely distended.[A balloon which is only half full at the surface of the earth, becomes quite fullwhen it has risen three miles and a half; because at that altitude, air from belowdoubles its volume on account of the diminished pressure.—Dr. Arnotts Elements. Fig, 44. 68 NATURAL PHILOSOPHY. of Physics. 3rd. ed. p. 40L]—If it were full at the time it left the ground, therewould be risk of its bursting open as it arose. The gas balloon requires a valve placedat its top, so that gas may be discharged at pleasure, and the machine made to aeronaut has control over its motions by taking up, with him a quantity of sandi}i bags, as ballast. If he throws out sand, the balloon rises, and if he opens the valve,and lets the gas escape, it descends. The rarefaction which air undergoes by heat makes it, of course, specifically air, therefore, ascends, and cold air descends. When the door of a room, whichis very warm, is open, the hot air flows out at the top, and the cold enters at the floor ;these currents may be easily traced by holding a candle near the bottom and top ofthe door. In the former position the flame leans inward, in the latter it is turnedoutward, following the course of the draught. The dr
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