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 GO 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.