to give it any value. The fact that the submersible is propelled under water by powerful electric motors begets the idea that the electrical disturbances therein might be detected by highly sensitive detectors of feeble electrical oscillations. The sea-water, in this case, will be found to absorb to a tremendous extent the effects of the electrical disturbance. Moreover, the metallic hull of the submersible forms in itself an almost ideal shield to screen the outgoing effect of these motors.

Considerable and important development has been made in the creation of sensitive sound-receiving devices, to hear the propeller vibrations and the mechanical vibrations that are present in a submersible, both of which are transmitted through the water. There are three principal obstacles to the successful use of such a device: when the submersible is submerged, she employs rotary and not reciprocating prime-movers, being in consequence relatively quiet when running under water, and inaudible at any considerable distance; the noises of the vessel carrying the listening devices are difficult to exclude, as are also the noises of the sea, which are multitudinous; finally, the sound-receiving instruments are not highly directive, hence are not of great assistance in determining the position of the object from which they are receiving sounds.[3]

To locate the submersible, aërial observation has been found useful. It is particularly so when the waters are clear enough to observe the vessel when submerged to some depth, but its value is less than might be supposed in the waters about the British Isles and Northern Europe, where there is a great deal of matter in suspension which makes the sea unusually opaque. The submersible, however, when running along the surface with only its periscope showing, is more easily detected by aircraft than by a surface vessel. Behind the periscope, there is a characteristic small wake, which is distinguishable from above, but practically invisible from a low level of observation. Many sea-planes are operating on the other side for the purpose of locating enemy submersibles and reporting their presence to the surface patrol craft. In order to overcome the disadvantages of creating the periscope wake which I have mentioned, it is reported that the Germans have developed special means to allow the U-boats, when raiding, to submerge to a fixed depth without moving. To maintain any body in a fluid medium in a static position is a difficult matter, as is shown in the instability of aircraft. One of the great problems of the submersible has been to master the difficulties of its control while maintaining a desired depth. The modern submersible usually forces itself under water, while still in a slightly buoyant condition, by its propellers and by the action of two sets of rudders, or hydroplanes, which are arranged along its superstructure and which tend to force it below the surface when they are given a certain inclination; but should the engines stop, the diving rudders, or hydroplanes, would become ineffective, and, because of the reserve buoyancy in the hull, the vessel would come to the surface.

In order to maintain the vessel in a state of suspension under water without moving, it would be necessary to hold an extremely delicate balance between the weight of the submarine and that of the water which it displaces. Variations in weights are so important to the submersible that, as fuel is used, water is allowed to enter certain tanks to compensate exactly for the loss of the weight of the fuel. To obtain such an equilibrium, an automatic device controlled by the pressure of the water, which, of course, varies with the depth, is used. This device controls the pumps which fill or empty the ballast-tanks, so as to keep the relation of the submersible to the water which it displaces constant, under which condition the vessel maintains a fixed depth. The principle of this mechanism is, of course, old, and was first embodied in the Whitehead torpedo, which has a device that can be set so as to maintain the depth at which it will run practically constant. With the addition of a telescopic periscope, which can be shortened or extended at will, it will be possible for the U-boat to lie motionless with only the minute surface of the periscope revealing her position.

IV

To attack the submersible is a matter of opportunity. It is only when one is caught operating on the surface, or is forced to the surface by becoming entangled in nets, that the patrol has the chance to fire upon it. Against this method of attack, modern submersibles have been improving their defenses. To-day, they are shielded with armor of some weight on the superstructure and over part of the hull. They are also equipped with guns up to five inches in diameter, and, affording, as they do, a fairly steady base, they can outmatch in gun-play any of the lighter patrol boats which they may encounter.

One of the important improvements which have been made has resulted in the increased speed with which they now submerge from the condition of surface trim. A submersible of a thousand tons displacement will carry about five hundred tons of water ballast. The problem of submerging is mainly that of being able rapidly to fill the tanks. On account of the necessity of dealing with large quantities of water in the ballast system, the European submersibles are equipped with pumps which can handle eight tons of water per minute.

Again, the speed which the electrical propulsion system gives the vessel on the surface greatly increases the pressure which the diving rudders can exert in forcing the submersible under water. This effect may be so marked that it becomes excessive, and Sueter emphasizes the point that vessels at high speed, when moving under water, may, on account of the momentum attained, submerge to excessive depths. To eliminate this tendency, there is a hydrostatic safety system which automatically causes the discharge of water from the ballast-tank when dangerous pressures are reached, thus bringing the submersible to a higher level where the pressure on the hull will not be so severe. From this it follows that the opportunity of ramming a submersible, or of sinking it by gunfire, is greatly minimized, since the vessel can disappear so rapidly.

Copyright by Underwood & Underwood, N.Y.

INTERIOR OF A SUBMARINEToList

A great deal has been attempted with nets. Fixed nets extend across many of the bodies of water around the British Isles. Their positions, doubtless, are now very well known to the Germans. The problem of cutting through them is not a difficult one. Moreover, the hull of the submersible has been modified so that the propellers are almost entirely shielded and incased in such a way that they will not foul the lines of a net. There has also been a steel