The parts indicated by numbers in this illustration are as follows: 1, main ballast tanks; 2, fuel tanks; 3, keel; 4, safety drop keel; 5, habitable superstructure; 6, escape and safety chambers; 7, disappearing anti-air craft guns; 8, rapid fire gun; 9, torpedo tubes; 10, torpedoes; 11, twin deck torpedo tubes; 12, torpedo firing tank; 13, anchor; 14, periscopes; 15, wireless; 16, crew’s quarters; 17, officers’ quarters; 18, war-head stowage; 19, torpedo hatch; 20, diving chamber; 21, electric storage battery; 22, galley; 23, steering gear; 24, binnacle; 25, searchlight; 26, conning-tower; 27, diving station; 28, control tank; 29, compressed air flasks; 30, forward engine room and engines; 31, after engine room and engines; 32, central control compartment; 33, torpedo room; 34, electric motor room; 35, switchboard; 36, ballast pump; 37, auxiliary machinery room; 38, hydroplane; 39, vertical rudders; 40, signal masts. ] “That’s where the gasoline is mixed with air, before it goes into the cylinder.” The engineer nodded. “The mixture is sucked into the cylinder by the down-stroke of the piston. The up-stroke compresses it, and then the mixture is exploded by an electric spark from the spark-plug. The force of the explosion drives the piston down, and the next stroke up drives out the refuse gases. That’s how an ordinary, four-cycle gasoline motor works. “But the Diesel engine,” he continued, “doesn’t need any carburetor or spark-plug. When the piston makes its first upward or compression-stroke, there is nothing in the cylinder but pure air. This is compressed to a pressure of about 500 pounds a square inch--and when you squeeze anything as hard as that, you make it mighty hot--” “Like a blacksmith pounding a piece of cold iron to a red heat?” I suggested. The engineer nodded again. “That compressed air is so hot that the oil which has been spurted in through an injection-valve is exploded, and drives the piston down on the power-stroke. The waste gases are then blown out by compressed air. There are an air-compressor and a storage tank just for scavenging, or blowing the waste gases out of every three power-cylinders.” “What are the advantages of the Diesel over the gasoline engine?” [Illustration: Courtesy of the Electric Boat Company. Auxiliary Switchboard and Electric Cook-stove, in a U. S. Submarine.]