Hydrogen is itself an inflammable gas, and when mixed with air or oxygen is dangerously explosive. It has, therefore, always been a source of great concern to balloonists, who had long dreamed of a non-inflammable, non-explosive gas, sufficiently light to function as does hydrogen. It was known that helium was such a gas, but it was, until very recently, so scarce and costly that its use in balloons had scarcely been given a serious thought. Not more than 100 cubic feet of helium had ever been produced up to the time we started our balloon programme, and it was valued at $1,700 a cubic foot. Scientific investigators in the employ of the government discovered about this time, however, that certain natural gases in the United States contained limited quantities of helium, and the problem then resolved itself into one of extracting the helium from these gases in sufficient quantities, and at a sufficiently low cost, to make practical its use. Funds were forthcoming and, under the supervision of the Navy Department and the Bureau of Mines, the process of gas liquefaction was put into operation, with the result that on the day of the Armistice there were on the docks, ready for shipment overseas, 147,000 cubic feet of helium with a pre-war value of a quarter of a billion dollars. Plants were under construction which, had the war continued, would have produced 50,000 cubic feet of this gas a day at a cost of approximately ten cents per cubic foot. The importance of this discovery cannot be overestimated, for it marks the opening of a new era in lighter-than-airship navigation. In war it will make the incendiary bullet, which has caused the destruction of countless balloons, a joke. The only way to bring down a balloon filled with helium will be literally to tear it apart by a direct hit with a high-explosive shell. Under peace conditions, it opens up undreamed-of possibilities in the development of new types of dirigible airships, as the danger from lightning, static electricity, and sparks of any kind has been entirely eliminated. To cross the Atlantic in a helium-filled balloon will be safer, so far as danger from fire is concerned, than to cross the continent in a train. * * * * *