"But how on earth do you manage to get your vapour up to that tremendous pressure?" asked the colonel. "Oh!" answered the professor, "that is a mere matter of mixing. According to the proportions in which the crystals and the acid are mingled together, so is the pressure of the vapour." "And how do you mingle them together?" asked the lieutenant. "This," said the professor, leading them up to a small boiler-like vessel, "is the generator. The crystals are placed in a hopper at one end, and the acid in that small tank at the other, from whence they are respectively conducted along tubes into a small well in the bottom of the generator, where, their proportions being regulated by the size of the tubes through which they pass, they mingle and generate a vapour having a pressure of five thousand pounds on the square inch. See, there is the gauge, and it is now registering a pressure of five thousand pounds." "Good Heavens, man!" exclaimed the baronet, starting back; "you don't mean to say that your generator is _now_, at this moment, subjected to that enormous pressure of more than two tons per square inch? Supposing it exploded, what would become of us?" "We should be consumed in an instant by the fierce heat of the liberated vapour," replied the professor calmly. "But," he continued, "you need have no apprehension of an explosion. When that generator was being made I had a second one constructed at the same time, precisely similar in every respect, and this second one I tested to destruction, with the satisfactory result that it endured without distress a pressure of thirty-five tons per square inch, showed the first signs of weakness when it became subjected to a pressure of thirty-eight tons, and burst at a joint when under a pressure of forty-three tons per square inch. You may therefore feel quite satisfied that the generator is fully equal to a continuous pressure of at least fifteen tons, instead of the trifle over two which it will have to sustain." The remainder of the machinery possessing no very startling or novel features, it was passed by with merely an admiring glance at its exquisite finish; and the quartette, leaving the engine-room, passed round on the other side of the spiral staircase to a room marked "Diving Room." Entering this they found themselves in an apartment about twenty feet square, one side of which was wholly occupied by four cupboards labelled respectively "Sir Reginald Elphinstone," "Colonel Lethbridge," "Lieutenant Mildmay," and "Von Schalckenberg." "This," explained the professor, "is the room wherein we shall equip ourselves for our submarine rambles; and here," opening one of the cupboards, "are the costumes which we shall wear upon such occasions." The opened cupboard contained an ordinary indiarubber diving-dress, a sort of double knapsack, a number of heterogeneous articles, and, lastly, a suit of armour.