The bearings of the wheel are supported upon a metal ring which floats upon the surface of a trough of mercury. Thus friction is brought down almost to the irreducible minimum. The only place where the wheel and its supports touch anything solid is at one delicately made pivot which serves to keep the floating mechanism in the centre of the mercury basin, and to prevent it from rubbing against the side of it. The current which drives the motor reaches it through this pivot and leaves through the mercury. Thus arranged, although the floating part is of considerable weight, a very slight force indeed is enough to move it; while, looking at it the other way, we can see that the ship might turn rapidly to right or to left, carrying round the mercury bowl with it, without turning the floating part at all. Thus the gyroscopic action is very free indeed to exercise its function of keeping the contrivance pointing always in the one way. The float has mounted upon it a compass card much like that of the ordinary magnetic instrument, and the sailor reads it in precisely the same way. To outward appearance there is little essential difference; in one case there is a magnet under the card to keep it still, in the other there is the float with the revolving wheel mounted upon it. It is customary to have one "master compass" of this kind on a ship, with an electrical repeater in each of the steering positions. As the "master" turns in its casing it sends a rapid series of currents to all the others, causing them to turn in unison with it. The "master" is fitted in some safe part of the ship where it is least likely to be the victim of any accidental damage. CHAPTER VIII TORPEDOES AND SUBMARINE MINES It is sad to think how much scientific skill and learning has, during the Great War, been devoted to killing people. It used to be thought that one day a great scientific invention would arise, of such deadly power that for ever afterwards war would be unthinkable; its horrors would be such that all nations would shrink from it. That prophecy, however, has not been fulfilled, nor are there any signs of it. On the contrary, each scientific achievement in the realm of warfare is quickly countered by another: so much so that with all our science in the manufacture of weapons, and our skill in using them, warfare in the twentieth century is if anything less deadly in proportion to the numbers engaged than it used to be. There are, however, two weapons which in this war have reached a deadly efficiency which they did not seem to possess before, and to which satisfactory antidotes have not yet appeared.