The main hull of the vessel was literally filled with machinery. In the after part of the boat were the Diesel oil-engines with which the U-boat was propelled when on the surface. There were two engines, each driving a propeller-shaft. It was impossible to use the engines when the vessel was submerged, not because of the gases they produced--these could easily have been carried out of the boat--but because every internal-combustion engine consumes enormous quantities of air. In a few minutes the engines would devour all the air in the hull of the submarine and would then die of suffocation. And so the engines were used only when the submarine was running awash or on the surface, and then the air consumed by them would rush down the hatchway like a hurricane to supply their mighty lungs. ENGINES THAT BURN HEAVY OIL The oil-engines were strictly a German invention. In the earlier days of the submarine gasolene-engines were used, but despite every precaution, gasolene vapors occasionally would leak out of the reservoirs and accumulate in pockets or along the floors of the hull, and it needed but a spark to produce an explosion that would blow up the submarine. But Rudolph Diesel, a German, invented an engine which would burn heavy oils. [Illustration: (C) Underwood & Underwood Complex Mass of Wheels and Dials inside a German Submarine] In the Diesel engine there are no spark-plugs and no magneto: the engine fires itself without electrical help. Air is let into the cylinder at ordinary atmospheric pressure, or fifteen pounds per square inch. But it is compressed by the upward stroke of the piston to about five hundred pounds per square inch. When air is compressed it develops heat and the sudden high compression to over thirty times its normal pressure raises the temperature to something like 1000 degrees Fahrenheit. Just as this temperature is reached, a jet of oil is blown into the cylinder by air under still higher pressure. Immediately the spray of oil bursts into flame and the hot gases of combustion drive the piston down. Because of the intense heat almost any oil, from light gasolene to heavy, almost tarlike oils, can be used. As heavy oils do not throw off any explosive vapors unless they are heated, they make a very safe fuel for submarines. [Illustration: Photograph by International Film Service Surrendered German Submarines, showing the Net Cutters at the Bow]