Many other devices to trap, sink or capture submarines have been invented. A large number of these, of course, have been found impracticable. Others, however, have been used with success. Few details of any of these have been allowed to become known.
The most dangerous power of submarines, is their ability to approach very closely to their object of attack without making their presence known to their prey. This naturally suggested that a way be found to detect the presence of submarines early enough to make it possible to stave off an attack or even to assume the offensive against the underwater boat. A recent invention, the perfection of which is due to the work of Mr. William Dubilier, an American electrical engineer, and of Professor Tissot, a member of the French Academy of Science, is the microphone. Few details are known about this instrument except that it records sound waves at as great a distance as fifty-five miles. This would permit in most cases the calling of patrol boats or the use of other defensive means before the submarine would be able to execute an attack.
At the present moment it would appear that the most dangerous enemy of the submarine yet discovered is the airplane or the dirigible. Some figures as to the mortality among submarines due to the efforts of aircraft have been published in an earlier chapter. The chief value of aircraft in this work is due to the fact that objects under the water are readily discernible at a considerable depth when viewed from a point directly over them. An illustration familiar to every boy is to be found in the fact that he can see fish at the bottom of a clear stream from a bridge, while from the shore the refraction of the water is such that he can see nothing. From the air the aviator can readily see a submarine at a depth of fifty feet unless the water is unusually rough or turbid. The higher he rises the wider is his sphere of vision. With the lurking craft thus located the airman can either signal to watching destroyers or may bide his time and follow the submarine until it rises to the surface, when a well placed bomb will destroy it. Both of these methods have been adopted with success. For a time the submarines were immune from this form of attack because of the difficulty of finding a bomb which would not explode on striking the surface of the water, thus allowing its force to be dissipated before it reached the submarine, or else would not have its velocity so greatly checked by the water that on reaching the submarine the shock of its impact would not be great enough to explode it at all. Both of these difficulties have been overcome. The new high explosives have such power, taken in connection with the fact that water transmits the force of an explosion undiminished to a great distance, that many of them exploding at the surface will put out of action a submarine at a considerable depth. Furthermore bombs have been invented, which being fired, not merely dropped from an airplane, will go through the water with almost undiminished momentum and explode on striking the target, or after a period fixed by the assailant. Other bombs known as "depth bombs" are fitted with flanges that revolve as they sink, causing an explosion at any desired depth.
About the actual achievements of the airplane as a foe to submarines there hangs a haze of mystery. It has been the policy of the Allied governments to keep secret the record of submarines destroyed and particularly the methods of destruction. But we know that a few have met their fate from bolts dropped from the blue. In The Outlook Lawrence La Tourette Driggs, himself a flying man of no contemptible record, describes the method and result of such an attack. After recounting the steps by which a brother airman attained a position directly above a submerged submarine preparatory to dropping his bomb, he says:
Down shot his plummet of steel and neatly parted the waters ahead of the labouring submarine. But it did not explode. I could see a whirling metal propeller on the torpedo revolve as it sank. It must have missed the craft by twenty feet.
Suddenly a column of water higher than my position in the air stood straight up over the sea, then slipped noiselessly back. By all that is wonderful how did that happen?
As we covered the spot again and again in our circling machines, we were joined by two more pilots, and finally by a fast clipper steam yacht. The surface of the water was literally covered with oil, breaking up the ripple of the waves, and smoothing a huge area into gleaming bronze. Here and there floated a cork belt, odd bunches of cotton waste, a strip of carpet, and a wooden three-legged stool. These fragments alone remained to testify to the corpus delicti.
"Philip," I said half an hour later, as the hot coffee was thawing out our insides, "what kind of a civilized bomb do you call that?"
"That bears the simple little title of trinitrotoluol; call it T. N. T. for short," replied Sergeant Pieron.
"But what made it hang fire so long?" I demanded.