It will be seen that Mr. Edison's inventive imagination was permitted a wide scope. He fairly reveled in the opportunity of attacking so many difficult problems and worked through the days and nights writh unflagging enthusiasm. He committed his business interests to the care of his associates, and during the two years of his work for the government kept in touch with his great business interests only by means of reports which were condensed to the utmost. In addition, for two successive winters, he gave up his regular winter vacation on his Florida estate, usually a source of great enjoyment to him. But it was all done willingly and without a word of regret or dissatisfaction so far as the writer's knowledge goes. Although we cannot take space to discuss the above items in detail, the reader will probably have a desire to know something of Edison's work in regard to the submarines. In view of the vast destruction of shipping, perhaps it is not an overstatement to say that the most vital problem of the late war was to overcome the menace of the submarine. Undoubtedly there was more universal study and experiment on means and devices for locating and destroying submarines than on any other single problem. The class of apparatus most favored by investigators comprised various forms of listening devices by means of which it was hoped to detect and locate by sound the movement of an entirely submerged submarine. The difficulties in obtaining accurate results were very great even when the observing vessel was motionless, but were enormously enhanced on using listening devices on a vessel under way, on account of the noises of the vessel itself, the rushing of the water, and so on. Edison's earliest efforts were confined to the induction balance, but after two months of intensive experimenting on that line he gave it up and entered upon a long series of experiments with listening devices, employing telephones, audions, towing devices, resonators, etc. The Secretary of the Navy provided Edison with a 200-foot vessel for his experiments, and in the summer and fall of 1917 they had progressed sufficiently to enable him to detect sounds of moving vessels as far distant as five thousand yards. This, however, was when the observing vessel was at anchor. The results with the vessel under way, at full speed, were not poor. Having pushed the possibilities along this line to their reasonable limit, Edison was of the opinion that this plan would not be practical and he turned his thoughts to another solution of the problem--namely, to circumvent the destructive operation of the submarine and avoid the loss of ships. He had discovered in his experimenting that the noise made by a torpedo in its swift passage through the water was very marked and easily distinguishable from any other sound.