"Under the grating, Mr. Cobb." Mr. Lochridge raised the grated flooring, and showed three iron cylinders, each divided into halves, with piston-rods and cylinder-heads. They were about four feet in diameter by twenty-three feet long. "These, gentlemen," he continued, "are connected by pipes with the outside of the vessel. Water can be admitted into any one or all of these cylinders, and, in two minutes, driven out by the pistons. Should these pistons fail, from any cause, to work, pumps connected with the cylinders could perform the same duty in ten minutes. I gave you the weights a few minutes ago; what did I make them?" taking a piece of paper and pencil from his pocket, and making a few notes. "Yes; 1,940,000 pounds, or just thirty tons less than our displacement. The water cylinders have a capacity of fifty tons. By allowing thirty tons of water to enter the cylinders, our weight is equal to our displacement, and we sink. Allowing all loss of weight aboard ship during a cruise, and which never exceeds twenty tons, we can always decrease our buoyancy and sink to the bottom, if necessary. Now, here," pointing to the left, and along the walls of the vessel, "are the dynamos for the electric lights, fans for circulating the fresh air, steering apparatus, electric heaters, exhaust pumps for expelling the vitiated air and drawing in the fresh, and many other inventions, the uses of which you can learn at your leisure." The engine-room of the Tracer was indeed a curiosity-shop to Junius Cobb. Pipes in every direction; electric wires crossed and recrossed one another; peculiar machines occupied each side of the room, and a hundred other things, strange to him, were upon either side. Leaving the engine-room, Lieutenant Sibley led the way to the instrument-room of the ship. Here a new treat awaited Cobb. Situated just at the junction of the main shell and the forward cone, was the pilot's, or instrument, room. In an easy-chair, in front of a box about two feet square, and resting on the table, sat Mr. Irwin, the first pilot of the Tracer. On either side of him, and fastened to the walls of the room, were a great number of delicate instruments, some of which were familiar to Cobb. At either side of the box on the table were several rows of push-buttons; to the left, a fine compass, and to the right, speaking tubes and bells. "You met Mr. Cobb at breakfast, did you not, Irwin?" questioned Lieutenant Sibley, as the pilot arose and greeted the entrance of the party with a smile. "Yes, I had that pleasure," he returned, bowing. "Have you been over the ship?" to Cobb. "We have taken it all in, Mr. Irwin," said Lyman, answering for the party. "How is the course? and where are we now?" asked the Lieutenant. "It is now 9:35, and we are headed northeast by east. Cairo is to our rear ninety-five miles. We are over Princeton, thirty miles north of Evansville," was the reply. "You may make Louisville. What time will we get there?"