As a means of aerial navigation, in the proper sense of that term, the balloon is now generally acknowledged to be useless or worse than useless; but as an instrument for observation, whether in the operations of war or the purposes of meteorology, it is of great value. And as small captive balloons are easily constructed, at small expense, they will no doubt come into more general use in the future. A balloon eight feet in diameter, filled with gas, will carry up quite a number of efficient instruments for recording temperature, pressure, electrical condition, etc.
The art of ballooning depends upon the difference between the weights of the same bulk of air and that of some of the lighter gases. Thus pure hydrogen, weighed under similar conditions, is about 16 times lighter than common air; but when prepared on the large scale, and containing water, air, and other impurities, it is only from 7 to 11 times lighter than the atmosphere. A cubic foot of atmospheric air at the level of the sea weighs .07609 lb.; a similar globe of hydrogen (reckoning it only as 6 times lighter than common air), will therefore have an ascensional force of .063 lb., or rather more than an ounce. Now, the weight of the body of air which a balloon displaces must exceed the gross weight of the balloon, its contents, and all its appendages, in order for the latter to ascend in the atmosphere. The difference of the two weights expresses the ascensional force. 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 carburetted hydrogen supplied by the gasworks 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 best adapted for aerostation.
The aerostatic power of balloons is proportional to their dimensions in the ratio of their contents, and this is as the ratio of the cubes of their diameters. Thus it will be found 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 li foot in diameter will barely float, owing to the less proportionate volume of gas to the weight of the case containing it.
The following table shows the relations between the diameters, surfaces, and capacities of spheres: -
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 basket-work. Fire-balloons, on the small scale, are generally made of tissue-paper, 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.
Owing to the increasing rarity of the atmosphere as we ascend from the earth's surface, balloon-cases are made very much larger than is required to contain the necessary quantity of gas, to allow for its expansion as it rises into a rarer medium. A cubical foot of gas measured at the level of the sea occupies a space of two feet at an elevation of 3 1/2 miles.
M. Delon, of Paris, produces miniature balloons by means of ordinary gas conducted through a caoutchouc tube and clay pipe to glycerine soap solution. A small disk of thin paper, with fine wire from its center to a little paper car with aeronaut figures, is connected to the bubble when it begins to swell, the disk being attached by capillarity to the part where the drop forms. The detached bubble rises with its car.