To the captain's left stood a man whose job it was to hold the sub to the depth of water desired. This was the diving-rudder man, a most expert one, we were told, who had been known to hold a submerged sub at full speed to within six inches of one depth for two miles at a stretch. A thin brass scale and a curved tube of colored water with an air bubble in it helped out the diving-rudder man's calculations. The least deviation of the sub's course from the horizontal and these two instruments, lit up by electric lamps, showed it at once. There was a big dial, with a long green hand, which also marked the depth of the sub; but that was an insensitive and rather slow-acting gauge--all right for the crew to look at from half the length of the sub, but not fine or quick enough for the diving-rudder man. He was the busy man while we were under water. The others could now and again grab a moment of relaxation from their tenseness, but while the sub was moving the diving-rudder man never took his eyes off the little brass scale with the electric light playing on it. Stop and consider that our sub had only to get a downward inclination of ever so little while running hooked-up under water, and in no time she would be below her lowest safety depth of 200 feet, where the pressure is 7 tons to every square foot of her hull. And should she collapse there would be no preliminary small leak by way of warning. She would go as an egg-shell goes when you crush it in your palm. Plack!--like that--and it would be all over. Above this same middle compartment, the smallest and most crowded of all, up through the grilled spaces of a steel grating, we could see the wide feet and boot-legs of the man who held the ship to her compass course; and for a wheel, we knew, he was holding a little metal lever about as long and thick as his middle finger, with a little black ball about as big as the ball of his thumb on the end of it. To the right of the foot of the conning-tower ladder stood the ballast-tank man; and when the captain from the foot of his periscope gave the word--after first looking forward, aft, and to each side of him to see that all hands were at their proper stations--it was the ballast-tank man who went violently at once into action. He grabbed a big valve and gave it a twist; grabbed another and gave it a twist; and another, and one more; and, standing near by, we could hear--or thought we could--the in-rush of great waters. [Illustration: In the engine-room of a submarine. The Diesel engines, driven by crude petroleum, propel the ship on the surface. Electric motors supply the power when running submerged.]