In aërostation, a bag or hollow pearshaped vessel, made of varnished silk or other light material, and inflated with some gas or vapor lighter than the air, as hydrogen, carbureted hydrogen, heated air, etc., so as to rise and float in the atmosphere. When filled with gas it is called by way of distinction an AIR-BALLOON (aérostat, etc., Fr.; luftball, luft-schiff, etc., Ger.); when with heated air a FIREBALLOON or MONTGOLFIER B. (balloon à feu, etc., Fr.).

In the early days of aërostation, and indeed for some years afterwards, balloons were inflated with hydrogen gas, obtained by the action of sulphuric acid and water on iron filings or small fragments of iron; but this method of filling them ultimately gave place to the cheaper and more convenient supply afforded by the gas-light companies. Of late years, the coal-gas furnished by the gas-works has been generally, if not solely, used for the inflation of balloons.

The principles of ballooning may be referred to the well-known difference in the specific gravity of bodies, and to the physical properties of the atmosphere. Pure hydrogen, weighed at the bottom of the sea, is about 16 times lighter than common air; but when prepared on the large scale, and containing water and other impurities, it is only from 7 to 11 times lighter than the atmosphere. A globe of atmospheric air 1 foot in diameter, under like circumstances, weighs 1-25 lb.; a similar globe of hydrogen (reckoning it only as 6 times lighter than common air), will, therefore, have an ascensional force of 1-30 lb. Now the weight of the body of air which a balloon displaces must exceed the gross weight of the balloon and all its appendages, in order for the latter to ascend in the atmosphere. The difference of the two weights expresses the ascensional force. The aërostatic power of balloons is proportional to their dimensions, in the ratio of the cubes of their diameters. Thus, it appears that a balloon of 60 feet diameter filled with common hydrogen will ascend with a weight of nearly 7,000 lbs., besides the gas case; whilst one of only 1% feet in diameter will barely float, owing to the less proportionate volume of gas to the weight of the case containing it. In round numbers the buoyancy of a balloon may be

reckoned as equal to 1 oz. for every cubic foot of hydrogen it contains, less the weight of the case and appendages. The carbureted hydrogen supplied by the gas-works is much heavier than hydrogen gas, and consequently much less buoyant, for which due allowance must be made. That which possesses the least illuminating power is the lightest, and consequently the best adapted for aërostation.

The fabric of which the cases of air-balloons are made is strong thin silk, covered with an elastic varnish of drying oil or india-rubber, or, what is better, a solution of india-rubber in either chloroform or bisulphide of carbon the netting is of strong light silk or flaxen cord; and the car, of basketwork. Fire-balloons, on the small scale, are generally made of silverpaper, and are inflated with the fumes of burning alcohol by means of a sponge dipped in that liquid, and suspended just within the mouth of the apparatus.

The following table will prove useful to the amateur aëronaut or balloonist:

No motive power machine sufficiently light and powerful to lift itself from the ground and maintain itself in the air for any considerable time has yet been invented. Aerial navigation is therefore at present limited to the use of balloons filled with light gas or hot air. Common coal gas is found to be the cheapest and most generally available gas for ballooning. 1,000 cubic feet of coal gas will lift 35 pounds weight. But hydrogen is the best gas for the purpose. 1,000 cubic feet of hydrogen gas will lift from 60 to 70 pounds. It is the lightest of all substances. It is fifteen times lighter than air, and over eleven thousand

times lighter than water. One of the cheapest ways to make hydrogen for balloons is to dissolve zinc in sulphuric acid; the latter is composed of sulphur and hydrogen. When the acid is poured on zinc, the sulphur unites with the metal and sets free the hydrogen, which bubbles up, and is conducted in a pipe to the balloon. Various efforts to propel and steer balloons have been made, by means of propellers turned by hand; also by the use of the electrical storage battery. Balloons are generally made of cotton cloth or silk, varnished with linseed oil, and dissolved rubber is sometimes mixed with the oil.