For this purpose there is first of all a "hydrostatic valve." This little appliance, which is open to the action of the water, responds to changes in pressure. The pressure at any point under water is exactly proportional to the depth. At ten feet, for example, it is precisely ten times what it is at one foot. So the hydrostatic valve is adjusted to set the rudders straight when the water-pressure upon it is a certain amount. If, then, it dives downwards the pressure increases and the valve operates the rudders so as to bring it upwards, while if it rise too high the decrease of pressure causes it to be guided downwards. This action, however, is too sudden and violent, so that with it alone the torpedo would proceed by leaps and bounds. After being low it would come up too suddenly, overshoot the mark, only to be steered downwards again equally suddenly. The valve, therefore, is combined with a pendulum, whose action tends to restrain these too sudden changes, with the result that under the influence of the two things combined the torpedo keeps fairly well to an even course, only varying upwards or downwards to an extent which is negligible. Finally, there is an interesting little feature about the firing mechanism which merits a description. The actual firing is caused by the driving in of a little pin which projects at the nose of the torpedo. Suppose that, in the process of pointing the torpedo and launching it upon its course, that pin were to be knocked accidentally, an awful disaster would result. It must be provided against, therefore, and the method adopted is beautiful in its certainty and simplicity. Normally, the firing-pin is fixed by a screw so securely that no accidental firing is possible. There is, however, a little propeller-like object associated with it, which is driven round by the water as the torpedo is pushed through it, and this unscrews, and thereby releases the pin. The little "fan" has to rotate a certain number of times before the pin is released, and it is quite impossible for this number to be accomplished before the torpedo has proceeded to a safe distance from the ship which fires it. On board the ship, therefore, and so long as it is near the ship, it is quite safe, but by the time it reaches its target it is ready to explode. As far as is known, the foregoing description gives a true general description of the torpedoes now in use. Those of different powers may vary in detail, but, broadly, they are as just described. There are others, however. The Brennan, for instance, was once adopted and largely used by the British for harbour defence. This was controlled from the shore by wires. It was driven, so to speak, with wire reins, and thus guided it could fairly hunt down its prey, turning to right and to left as required.