On one occasion a young German military attaché begged the privilege of making an ascent in the balloon. Permission was given and when the German officer returned to earth he was wildly enthusiastic in praise of this aërial observation post. He had had a splendid view of the enemy and could watch operations through his field-glasses which were of utmost importance. Realizing the military value of the aircraft, he returned to Germany and urged military authorities to provide themselves with captive balloons. This young officer was Count Ferdinand von Zeppelin, who was destined later to become the most famous aëronautic authority in the world and who lived to see Germany equipped with a fleet of balloons which were self-propelling and could travel over land and sea to spread German frightfulness into England. He also lived to see the virtual failure of this type of war-machine in the recent great conflict, and it was possibly because of his deep disappointment at having his huge expensive airships bested by cheap little airplanes that Count von Zeppelin died in March, 1917. However, he was spared the humiliation of seeing a fleet of Zeppelins lose their way in a fog and fall into France, one of them being captured before it could be destroyed, so that all its secrets of construction were learned by the French. THE WEIGHT OF HYDROGEN Before we describe the Zeppelin airships and the means by which they were eventually overcome, we must know something about the principles of balloons. Every one knows that balloons are kept up in the air by means of a very light gas, but somehow the general public fails to understand why the gas should hold it up. Some people have a notion that there is something mysterious about hydrogen gas which makes it resist the pull of gravity, and that the more hydrogen you crowd into the balloon the more weight it will lift. But hydrogen has weight and feels the pull of gravity just as air does, or water, or lead. The only reason the balloon rises is because it weighs less than the air it displaces. It is hard to think of air as having weight, but if we weigh air, hydrogen, coal-gas, or any other gas, in a vacuum, it will tip the scales just as a solid would. A thousand cubic feet of air weighs 80 pounds. In other words, the air in a room ten feet square with a ceiling ten feet high, weighs just about 80 pounds. The same amount of coal-gas weighed in a vacuum would register only 40 pounds; while an equal volume of hydrogen would weigh only 5-1/2 pounds. But when we speak of volumes of gas we must remember that gas, unlike a liquid or a solid, can be compressed or expanded to almost any dimensions. For instance, we could easily fill our room with a ton of air if the walls would stand the pressure; or we could pump out the air, until there were but a few ounces of air left. But in one case the air would be so highly compressed that it would exert a pressure of about 375 pounds