Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . o generatehydrogen, but they are either dangerous or costly or these may be mentioned the reaction between slakedlime and zinc, between steam and fused zinc, between sodiumand water, or potassium and water, and between zinc or aluminiumand either of the caustic alkalis. In any case the generation of the gas on the field of battlewould be out of the question, and the English method of usingthe compressed gas in steel cylinders is now ever
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . o generatehydrogen, but they are either dangerous or costly or these may be mentioned the reaction between slakedlime and zinc, between steam and fused zinc, between sodiumand water, or potassium and water, and between zinc or aluminiumand either of the caustic alkalis. In any case the generation of the gas on the field of battlewould be out of the question, and the English method of usingthe compressed gas in steel cylinders is now everywhere cylinder with walls 0*187 in. thick weighs about 88 lbs. and con-tains 140 cubic feet under a pressure of 120 or 130 military waggon carries 35 cylinders, and the gas is allowed to BALLOON CONSTRUCTION. ETC. 179 pass into the balloon by opening the valve at the top of thecylinder. When a balloon is to be inflated, several waggons aredrawn up at the side, and the various cylinders are all connectedto a tube, which conveys the gas to the interior of the inflation occupies from 15 to 20 Fig. 106.—Steel cylinder for containing hydrogen. Coal gas is only used for free balloons, and was first proposedby Green in 1818. The lift due to the use of the hydrogenor coal gas has been already studied in an earlier chapter, and itwas there shown that the size of the balloon depends on theamount of lift that is wanted. Therefore cap-tive balloons, which are generally filled withhydrogen, are much smaller than free balloonsfilled with coal gas. If it were not for thematter of expense the use of coal gas wouldcertainly be discontinued. The sphere is the body which combines thesmallest surface with the greatest volume, andtherefore all free balloons are spherical inshape. The size is obviously dependent on theweight of the load to be lifted. Generally speak-ing, a balloon to carry 3 or 4 persons would havea capacity of 45,000 cubic feet or t
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