surplus hydrogen had to be pumped into steel tanks where, owing to its compression, it was heavy and served as ballast which could at any time be fed back into the gas bags to increase the buoyancy of the airship. We can no longer think of air as having no weight when we consider that all this tonnage was supported by air. The R-34 was by no means the largest dirigible built, but we dare not boast of the size of the airships of to-day when the aeronautics is in its in-fancy, because our present dirigibles may seem puny alongside the big aircraft that may be built to-morrow. The dimensions of the R-34 have been given because of the historic interest in this first dirigible to span the Atlantic Ocean. HEAVIER-THAN-AIR MACHINES Marvelous as was the achievement of the Montgolfiers and wonderful as were the aeronautic developments that followed the invention of the balloon, the dominion of the birds was not really conquered until man had learned how to fly in a machine heavier than the air. Captive aeroplanes date back to the remote ages of ancient history. Kites are really “heavier-than-air machines.” They maintain themselves in the air because they travel through the air at a considerable velocity. True, a kite may be stationary, or practically so with respect to the ground, but if we detach ourselves from the ground and view the situation from a drifting balloon, the earth will appear to be moving under us and the kite will rush past us as it is dragged by the earth to which it is tied. The idea of propelling a kite through the air, not by tying it to the earth, but by furnishing it with its own propeller and power plant, was conceived long ago, but the problem was to find a power plant light enough. The honor of being the first man to rise off the ground in an aeroplane belongs to C. Ader, who made several short flights between 1890 and 1896 in a machine driven by a twenty-horsepower steam engine. Our own S. P. Langley did some most important pioneer work in flying and built a man-lifting, steam-driven machine in 1903 which would have flown had it not been for an accident to its launching gear. In fact, this very model was flown successfully a number of years later. However, it was not until the gasoline engine was developed that the power plant problem was solved. The internal-combustion motor was made more and more powerful in proportion to its weight until now there are several types that weigh less than two pounds per horsepower. But the power plant was only one obstacle to be overcome. The real problem was to learn how to control the machine after it rose into the air. Otto Lilienthal attempted to learn how to fly in a motorless flying machine. He provided himself with wings and, jumping off a height or running down a slope, depended upon gravity to furnish him with the necessary propulsion through the air. Unfortunately after five years of gliding experiments a fatal accident terminated his aeronautic research. THE WRIGHT BROTHERS