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 They all use and must use such engines and they are usually though not necessarily of the Diesel type. To answer the requirements of a submarine its engines should be capable of being started in a moment and stopped in a moment and this condition is satisfied by the internal combustion engine alone. But though the value and efficiency of submarines are due to the use of the internal combustion engine it cannot claim the distinction of having made them possible. This belongs to a scientific invention of a yet more peaceful type. I well remember meeting at the Electrical Congress of Paris in 1881 a pale delicate looking man named Gaston Planté who took me into his laboratory to shew me the results of his patient work for years past. He had found that he could electrically oxidize the surface of lead plates in two degrees one more highly oxidized than the other and that when these were used as the plates of a galvanic battery they produced a current—the more highly oxidized plate losing some of its oxygen to the other. But the important part of 36