Capth,e13alloons.—To be sure of the course the balloon would take, it was customary, before making a voyage, to send up small balloons, and to wait until they took the desired direction. Aeronauts, however, frequently risked the experiment of findin,g an altitude where the direction of the wind would afford the required horizontal motive-force, the opinion being held that, according to observations (which, generally speaking, have proved correct), air-strata at different altitudes move in the most varying directions. These experiments have not, however, been uniformly successful. Even in making a simple ascension the place of landing could never be deter mined previously with any certainty; still less could the balloon be kept suspended at a certain point, which would have been very desirable for making scientific observations. Finally, to effect the descent, the costly gas had to be discharged and wasted when its retention might have been of great advantag,,-e. The attempt has recently been made to obtain all these favorable conditions by holding the balloon captive.
Strictly speaking, the balloon used for military purposes (fi/. 6r, fig. 6) is a captive balloon; but the purpose in view is to consider balloons which are held captive and are let up and drawn down by machine-power. Be sides its own weight, such a balloon must carry the occupant of the car and the weight of a long,- cable reaching to the earth. Hence, under otherwise equal conditions, its carrying-power must be considerably greater than that of a free balloon, and it must, therefore, either be made more voluminous or be filled with a lighter gas—that is, with hydrogen instead of illuminat ing gas.
Gyard's combining both these expedients, the captive bal loon constructed in 1S69 in Loudon (p/. 6r, fig. 5) was made a giant carry ing an oninibus-like gondola. Its diameter was 120 feet and its volume not less than 425,000 cubic feet. It was twice as large as " Le Geant," previously mentioned, and two and two-fifths times as large as the one at the Paris Exhibition of 1867. The filling of such a large balloon with illuminating g.,as being very costly, and with hydrogen gas still more so, its constructor, Giffard, was very successful in his endeavors to prepare as light an envelope as possible, using two layers of linen and onc of muslin, with layers of eaoutclionc between them, and over all seven layers of caout chouc and oil varnish. By this the weight of the envelope alone was increased 616o pounds, but the fabric was so impermeable that for weeks no addi tional gas had to be supplied. The balloon was placed in a canvas-covered frame building, and was secured to a cable about 2roo feet long, which was carried over a giiide-pulley, placed underground, to the drum of a steam windlass. In ascending, the balloon unwound the cable, and was drawn down by it without the necessity of discharging any gas. Financially the enterprise was not a success, especially as, after having been in use but a few weeks, a heavy storm and the carelessness of the eng,ineer caused the breaking of the cable and the escape of the " captive," which had to be captured and refilled. Similar enterprises have been attempted, and with
considerable l'cla1, at several more recent exhibitions.
The relation of a captive balloon to the currents of the wind differs en tirely from that of a free balloon. While the latter yields to the shocks and pressures of the storm and the aeronaut scarcely feels them, every shock of the wind is disagreeably felt in the captive balloon; and if it be not quickly hauled down by the cable, it is liable to get into the critical situation already described, and illustrated in Figure 4 (fii. 6o).
Dirigible after the invention of the balloon it was confidently believed that means for its successful guidance could readily be devised, and consequently sails and rudders (with which Blanchard's bal loon,fig. 3, was provided) were at first employed. But these naturally proved useless, since, instead of combating the wind, they presented to it additional points of attack. A vessel on river or sea is able to advance against the wind by means of the sails, since tacking, can be effected by the rudder operating against the resistance of the water.
Similarly, steering, after a fashion, may be accomplished in aerostation with the help of a resistance supplementing that of the air. A project of this kind by an anonymous aeronaut is represented in Figure 7 (Ai. 61). The balloon, provided with sails and secured to a rudder floating, in the sea, is sufficiently distended to tighten the towing-ropcs in a calm without raising the rudder to any extent. With a rising wind, it is moved forward with the assistance of the sails. By placing the rudder, by means of the towing lines, parallel to the direction of the wind, the balloon, with but a small resistance to overcome, will be moved exactly in the direction of the wind. By placing the rudder horizontally at right angles to the course of the wind, the balloon is also moved in the direction of the wind, but more slowly, be cause of the greater resistance afforded by the rudder. By placing it, how ever, obliquely to the direction of the wind, the balloon must take a course deviating more or less from the direction of the latter; and hence to a cer tain extent the direction and the rapidity of its motion are under control. On account of the artificial Iesistance the balloon receives, however, a wind pressure of considerable energy might readily bring about a catastrophe like that mentioned in describing the captive balloon (pi. Go, fig. 4). Moreover, by giving a special form to the balloon, a species of steering—that is, the bringing about of a movement in another direction than that of the wind —may also, without aid from any external resistance, be effected by the resistance of the air itself. The term " fish-shape " or " cigar-shape " is applied to such forms as are illustrated in Figure 9.