One of the most important scientific developments of the war was the result of the endeavor of the American Air Service to find a non-inflammable gas for balloons. Investigation and experiment by the United States Bureau of Mines found a new source for helium in a natural gas field in the Southwest, from which it could be produced so cheaply as to make possible its use for this purpose. Up to that time no more than a few hundred cubic feet had ever been obtained and its value was $1,700 per cubic foot. When the war ended 150,000 cubic feet of helium for balloon inflation had been shipped and plants were under construction that would produce 50,000 cubic feet per day at a cost of about ten cents per foot. As a helium filled balloon could not be destroyed by incendiary bullets it would be comparatively safe from enemy attacks and could carry on over the enemy lines operations of the greatest importance. Both the American and British governments had perfected their plans, when the armistice was signed, to use many dirigibles thus filled in air attacks from which immense quantities of bombs would have been dropped over strategic points in Germany. Because of the assurance of safety which this non-inflammable gas gives to balloon operations, the usefulness of all balloons, but especially of the dirigible type, has been enormously increased and a new era opened for their service. Working upon the problem of making it possible to send propaganda balloons upon long journeys over the enemy’s country, the meteorological service developed ingenious types of balloons that did remarkable work of that kind during the last months of war and, in addition, give promise of very great usefulness for the days of peace. CHAPTER XIX FLYING IN FRANCE Seventeen large flying fields, divided into seven or more air instruction centers, one of which was the largest in the world, were developed in France for the partial training or final grooming of the men who had already received part or nearly complete preparation at the home fields. During the first year of the war 50,000 enlisted men were sent overseas to rush forward the preparations for our air forces. Most of them went to France, where they made ready the big flying fields at the instruction centers, built assembly depots for American-made planes and, later on, aerodromes near the front. Others were formed into service squadrons and trained in England and France, in order to lessen the pressure upon our hastily developed facilities for such training, and were held in readiness for work with American pilots. Still others took the places in factories of French and English workers in order to release those who were more highly skilled for specialized work on airplanes and their accessories.