I make the supposition that hydrogen, and not helium, will be the sustaining gas. For commercial aviation it has many advantages, for helium is dearer and rarer, and has about twenty per cent. less lift. Contrary to general belief, a flight in an airship filled with hydrogen, subject to proper precautions, has no greater fire risk than living near a gas factory. Helium is a necessity only for airships used in war, as, unlike hydrogen, it is not ignited by incendiary bullets from hostile aircraft. The United States has almost a monopoly of the world's quantitative supply of helium, which fact should be a tremendous asset in wartime. The ballast difficulty can be met by apparatus to condense the water of combustion from the exhaust gases of the engines. Experiments have shown that it is practicable to recover water of slightly greater weight than the gasoline fuel consumed, thus avoiding any variation in lift due to gasoline consumption. Further, water ballast could be picked up periodically from the sea by descending and taking in water through a pump suspended from a flexible hose, or direct into tanks in the gondolas through sea-valves. Still further reduction of running costs may be effected by fuel economy. This would be difficult with internal combustion engines of the type in use at present, for greater thermal efficiency (the ratio between the amount of heat contained in the fuel consumed and the amount of useful work delivered by the engine) necessitates heavier machinery. The reduction in gasoline consumption is thus offset by a decrease in the disposable lift. It is probable that a saving on large dirigibles might result from substituting for the internal combustion method of generating power engines that burn cheap oils. Although such engines are much heavier, and although the crude oils weigh a good deal more than gasoline, the difference would be more than covered on long flights, for gasoline is nearly four times dearer than crude oil. Moreover, the weight of oil actually consumed would be about twenty per cent. less than that of the gasoline burned by internal combustion engines over the same distance. The solution may be in the employment of steam. For the rather low standards of horsepower on which dirigibles are driven, heavy steam engines of the ordinary type, although much more reliable, would be less economical than internal combustion engines, owing to the latter's better thermal efficiency. Engineers are attempting to evolve a light type of steam turbine that will overcome this drawback. Of equal importance to fuel economy is a better system of airship navigation. This is similar in principle to steamship navigation, but it is made more complicated by the much greater drift of atmospheric currents. Moreover, air currents can never be charted as exactly as sea currents. An excellent meteorological organization, for reporting motions of the air at given times, is therefore essential.