ARROSTATION.
While the Plates devoted to the subject of locomotion represent vehicles that have been proved upon the touchstone of practical application, Plates 6o and 61 illustrate those inventions xvhich, regarding their mode of action as motive carriag-es, have thus far achieved only indifferent success.
The ascent of balloons and their movement by and with the winds no more solve the problem of aerostation than the launching- and rocking, of a rudderless and oarless boat tossed by waves and driven by currents solve the problem of navig-ation. The desire to move in the air like the bird, to strike out at will in this or that direction, and to keep a predeter mined course as the locomotive on the railroad or the vessel on the river or sea, has long- been cherished, but still remains unsatisfied. The attempts to realize this desire have been made chiefly in two directions. The first has been to take as a model the flight of birds, bats, and insects, and to construct flying-machines; and the second, to imitate the swimming of fish and to make the balloon guidable.
AOrostats. —The ordinary balloon, which is given a more or less spher ical, egg-, or pear-shaped form, rises vertically in a calm, and when there are air-currents ascends in a curved track which, with increasing height, approaches a horizontal line. At a certain elevation it remains suspended or moves in the direction of the prevailing wind. That this phenomenon is caused by the difference of the specific gravity of the filled balloon and the quantity of air displaced by it, and that it is identical with that which is presented by the motion of a piece of wood, or other object specifically lighter than water, ascending from the bottom of the sea, was known before aerostation became a science.
The Invention of the Balloon is claimed for the Portuguese Don Guzman, who in 1736 is said to have made an ascension in a paper balloon filled with " fire-air," though the missionary Vasson is authority for the assertion that a balloon ascended in Pekin in the year 1306. The invention, however, is commonly assigned to the year r -783, when the brothers Stephen and Joseph Montgolfier of Annonay, near Lyons, France, made a practical demonstra tion of the capability of a light vessel filled with heated air to rise freely in the surrounding atmosphere.
Scheme of Francis the discovery, in 1643, by Torricelli, of the g-ravity of the air, with which the invention of the balloon was evi dently connected, a scheme for aerial navigation was proposed by a Jesuit, Francis Lana, in a work entitled Proa'romo Arte Maestro' (Brescia, 16-7o). He suggested the employing of four very large copper balls or globes so thin that when exhausted of air they would readily rise. Each globe was to be about 25 feet in diameter and ,-.12-T of an inch in thickness; the four balls together would have an ascensional force of about twelve hundred pounds, which would be sufficient to raise the sails with which the machine was to be furnished, together with the gondola and its occu pants. As he proposed to exhaust the air by producing a Torricellian vacuum, he was evidently ignorant of the invention, in 1648, of the air pump by Otto von Guericke. In 1678, Sturm of Altdorf, near Nuremberg, also pursued this idea, and after Cavendish's discovery of hydrogen gas, in 1766, Professor Black of Edinburgh clearly expressed the theory that, on account of the small specific gravity of this gas, a light envelope filled with it would ascend in air. This theory was confirmed in 1782 by the experi ments tried by Kratzenstein, Lichtenberg, and Cavallo with soap-bubbles filled with hydrogen gas.
Lana's and Sturm's ideas were, of course, practically of no value, since the pressure of the exterior air would immediately have caused the copper balloon to collapse. To resist the external pressure, a reservoir which is to be made lighter by pumping out the air will require such thick walls that its weight will far exceed the admissible Ihnit. But since the discov ery of hydrogen gas—which, while exerting a pressure equal to that of the atmosphere, weighs only one-fourteenth as much—it is remarkable that the idea of using a containing vessel made of some light substance like paper, instead of the soap-bubble employed for the purpose, did not suggest itsel f.