The ideal submarine would weigh about the same amount as the water surrounding her. That being a practical impossibility, before she submerges she is trimmed down by means of water ballast, but then starts her engines and uses her planes for descent in the same way as an aeroplane. The flooding tanks, as we have seen, are between the two hulls, and the hydroplanes are in pairs both forward and aft. The U-boat has been running on the surface propelled by her internal-combustion motors. Obviously these cannot be used when she is submerged, or the air in the ship would speedily be used up. When about to submerge, the German captain trimmed his ship until just afloat; actually he frequently cruised in this trim when in the presence of shipping, ready to dive if attacked. The alarm was then pressed, the engineer pulled out the clutch, the coxswain controlling the forward hydroplane put his helm down, the captain entered the conning-tower, the hatch was closed, and away the steel fish cruised about beneath the surface. The U-boat was now running on her electric batteries. By means of two periscopes a view was obtained not merely of the sea above, but also of the sky, so that surface craft and aircraft might be visible. The order would be given to submerge to say 10 metres. Alongside each of the two coxswains was a huge dial marked in metres, and it was the sole duty of these two men to watch the dials, and by operating a big wheel controlling each hydroplane maintain the submarine at such a depth. Horizontal steering was done also by a wheel, and course kept by means of a gyroscope compass, a magnetic compass in this steel ship with so much electricity about being out of the question. The batteries were charged while the submarine was on the surface by turning the oil engines into a dynamo by means of the clutch, the hour before dawn and the hour after sunset being favourable times for so charging.