The balloon, though the earliest and crudest means of getting up in the air, has not become obsolete. It has been in existence practically in its present general form for upwards of 500 years. Appliances have been added from time to time, but the big gas envelope enclosing a volume of some gas lighter than an equal volume of air, and the basket, or car, suspended below it, remain as the typical form of aerial vehicle which has not changed since it was first devised in times so remote as to lie outside the boundaries of recorded history. The common shape of the gas bag of a balloon is that of the sphere, or sometimes of an inverted pear. It is allowed to rise and float away in the air as the prevailing wind may carry it. Attempts have been made to steer it in a desired direction, but they did not accomplish much until the gas bag was made long horizontally, in proportion to its height and width. With a drag-rope trailing behind on the ground from the rear end of the gas bag, and sails on the forward end, it was possible to guide the elongated balloon to some extent in a determined direction. In explaining why a balloon rises in the air, it is customary to quote the “principle of Archimedes,” discovered and formulated by that famous philosopher centuries before the Christian era. Briefly stated, it is this: Every body immersed in a fluid is acted upon by a force pressing upward, which is equal to the weight of the amount of the fluid displaced by the immersed body. It remained for Sir Isaac Newton to explain the principle of Archimedes (by the discovery of the law of gravitation), and to show that the reason why the immersed body is apparently pushed upward, is that the displaced fluid is attracted downward. In the case of a submerged bag of a gas lighter than air, the amount of force acting on the surrounding air is greater than that acting on the gas, and the latter is simply crowded out of the way by the descending air, and forced up to a higher level where its lighter bulk is balanced by the gravity acting upon it. The fluid in which the balloon is immersed is the air. The force with which the air crowds down around and under the balloon is its weight--weight being the measure of the attraction which gravity exerts upon any substance. The weight of air at a temperature of 32° Fahr., at the normal barometer pressure at the sea-level (29.92 inches of mercury), is 0.0807 lbs. per cubic foot. The gas used to fill a balloon must therefore weigh less than this, bulk for bulk, in order to be crowded upward by the heavier air--and thus exert its “lifting power,” as it is commonly called. In practice, two gases have been used for inflating balloons--hydrogen, and illuminating gas, made ordinarily from coal, and called “coal gas.” Hydrogen is the lightest substance known; that is, it is attracted less by gravity than any other known substance, in proportion to its bulk.