The quarter-master transfers himself to the steering wheel inside, where he has another compass to guide him, not of the magnetic variety this time but a cunning application of the gyroscope. The commander, too, having descended before the last hatch was closed down, takes his stand at the eyepiece of the periscope, since that is now his only means of seeing what is going on above. Another man takes his place at the wheel which controls the diving rudder, conveniently near to which is a pressure gauge so connected to the outer water that as the ship dives its depth is recorded upon its dial: that in effect is to him what the compass is to his comrade at the other wheel. With every movement of men there needs to be adjustment made to keep the ship on an even keel. Otherwise she would go down by the bow or down by the stern according as the men's weight shifted towards either end. This is arranged for by two small tanks formed in the structure of the vessel, one at either end. Connected together by pipes and controlled by compressed air, water can be transferred from one to the other at will and so the balance be always kept. Quite simple manipulations of a valve serve to accomplish this delicate balancing performance. It is perhaps not of such importance at this stage, but in a moment, when the whole vessel will be under water, a very little movement indeed will suffice to upset the equilibrium. Next water ballast is admitted into certain other spaces in the ship's structure, these spaces being called, because of the use to which they are put, ballast tanks. Gradually, as the incoming water increases the weight of the vessel, she sinks until she is awash. Then the diving rudders are set at the right angle (a pendulum serves to show the angle at which the boat points) and down she goes. As the pressure-gauge indicates the approach to the required depth the rudder is flattened out a little until just that position is found which keeps the boat under at the desired depth. Of course, when all hatches and openings were closed the supply of fresh air was cut off and after that the crew had to depend upon the air contained in the submarine. Also, they had to stop the engine, for without air it cannot work: nor can it work without giving off fumes, which, if admitted to the ship, would soon suffocate the crew. Just before closing up, therefore, the engine is stopped and electric motors take up the task of driving the ship.