“Each of those cylinders,” explained the captain, “contains liquid carbonic acid under high pressure. They are tapped one after another, and the escaping gas in the engine cylinders furnishes the motive power that drives the screws. I’ve been rather anxious all the way over lest one of them should explode, but they tell me the danger is much less than of an ordinary boiler explosion. The men in the engineers’ department were some of them afraid at first of being poisoned by escape of the gas. You will notice that the air down here is as fresh and pure as it is out of doors. We have the finest ventilating system I ever saw on any ship. A forced draught, supplied by fans driven by a small carbonic acid engine, changes the air in every part of the ship every three minutes. As a matter of fact, the chief engineer tells me there has been no leakage of gas for a moment during the voyage. The carbonic acid, just before it does its work of driving the pistons, is heated by an oil flame on the outside of the pipes through which it passes. This accomplishes two purposes. It increases the expansive power of the gas, and it makes it light enough to rise readily and escape through the smoke-stack. But we have other wonders. Come up on deck,” and the two men ascended to the main deck and walked to the rail. “Will you lighten ship, Mr. Walters?” said the captain to an officer who was passing. “Aye, sir,” replied the man as he disappeared. “Just notice carefully the distance between us and the water--about five feet I should say,” remarked the captain to his still silent companion. A moment later there was a queer humming noise somewhere below, and the ship’s frame seemed to tremble slightly. Nothing was said for a few minutes. Then the Englishman exclaimed: “What does it mean, captain? We are rising, or the water is falling.” “Just so. We are using the pressure of the carbonic acid to drive the water from our ballast compartments and lightening the ship. We have a double hull, and the space between the two skins is divided into sections. These sections are air or gas-tight. When they are filled with water, and we wish to empty them, we have only to open the plugs at the bottom by turning a rod running up to the deck, admit compressed gas above, and the water is quickly forced out. Then we close the plugs, shut off the gas, and the ship stands four or five feet higher out of the water than she did before. So you see we are able to regulate our draught, and we gain other advantages also. The two hulls are so constructed that they take up and dissipate the vibration which our high speed would otherwise render dangerous. We can make her light forward and heavy aft, or _vice versa_; in fact, we can adjust the ship to exactly the best conditions for the weather she happens to be in, and for the speed we wish to employ.”