The steam-engine built to give the motive power was perhaps the most interesting feature of the whole construction. Maxim employed steam in preference to any other power as being one with which he was most familiar, and yielding most force in proportion to the weight of the apparatus. He designed and constructed a pair of high-pressure compound engines, the high-pressure cylinders 5 inches in diameter, the low-pressure 8 inches, and both 1 foot stroke. Steam was supplied to the high-pressure cylinders at 320 lbs. per square inch from a tubular boiler heated by a gasolene burner so powerful in its action as to raise the pressure from 100 to 200 lbs. in a minute. The total weight of the boiler, burner, and engines developing 350 horse-power was 2000 lbs., or about 6 lbs. per horse-power. The two screw-propellers driven by the engine measured 17 feet 11 inches in diameter. The completed flying-machine, weighing 7500 lbs., was mounted on a railway-truck of 9-foot gauge, in Baldwyn’s Park, Kent, not far from the gun-factories for which Sir Hiram is famous. Outside and parallel to the 9-foot track was a second track, 35 feet across, with a reversed rail, so that as soon as the machine should rise from the inner track long spars furnished with flanged wheels at their extremities should press against the under side of the outer track and prevent the machine from rising too far. Dynamometers, or instruments for measuring strains, were fitted to decide the driving and lifting power of the screws. Experiments proved that with the engines working at full power the screw-thrust against the air was 2200 lbs., and the lifting force of the aeroplanes 10,000 lbs., or 1500 in excess of the machine’s weight. Everything being ready the machine was fastened to a dynamometer and steam run up until it strained at its tether with maximum power; when the moorings were suddenly released and it bounded forward at a terrific pace, so suddenly that some of the crew were flung violently down on to the platform. When a speed of 42 miles was reached the inner wheels left their track, and the outer wheels came into play. Unfortunately, the long 35-foot axletrees were too weak to bear the strain, and one of them broke. The upper track gave way, and for the first time in the history of the world a flying-machine actually left the ground fully equipped with engines, boiler, fuel, and a crew. The journey, however, was a short one, for part of the broken track fouled the screws, snapped a propeller blade and necessitated the shutting off of the steam, which done, the machine settled to earth, the wheels sinking into the sward and showing by the absence of any marks that it had come directly downwards and not run along the surface.