These were the kind of questions the captain believed in encouraging, though they sometimes led the conversation out of the legitimate channel. They elicited useful information; and he was careful not to let the students wander too wide of the subject under discussion. "I don't know now why a log or a fish goes best with the big end ahead," said Leo Pownall. "After the passage for a moving body in the water is opened, this fluid follows its own laws, and seeks an equilibrium. As it moves back to its natural level, it crowds in upon the after part of the body, whether it be a log, a fish, or a ship, and thus pushes it ahead. Under the stern of a vessel, the hull is curved, or hollowed out, just as the size of a fish diminishes at the tail, which is the fish's rudder. "But the shape of the hull is varied according to the use to which the vessel is to be put; but the rule will hold good in the main. In building a ship the beginning of the work is done on paper. As in erecting a house, the first thing is to obtain the plans, which are made by the naval architect. In fact, the entire shape of the vessel is laid down on the drawing-board. From these the builder gets his dimensions, all the curves, and the form of every timber and piece of wood used. "On the drawing on the wall," continued the principal, taking the pointer and indicating the plans, "everything is put down that can be needed in the construction of the boat we intend to build. There are three plans, you will observe. I had them drawn by a naval architect in New York. This," and the principal pointed to the highest one on the paper, "is the sheer plan. It shows the side or profile of the hull on a flat surface. It looks just as the broadside of the Sylph would, if she were too far off for you to get any idea of the curves in her sides. "This plan gives the exact curve of the bow, and the exact slant of the stern-post. The three straight lines extending the whole length of the hull are the levels to which the water would stand if the vessel were submerged to three different depths. This drawing is made on a scale of one inch to a foot. The sheer plan is a vertical plane through the keel. From it we get the length and height. The red lines which extend from certain points at the bow to the lower part of the stern post indicate the various curves of the hull at different distances from the vertical plane of the keel. In other words, they are three vertical planes, parallel with the central plane. "The next plan, of the same length as the first, shows you one half of the deck of the boat, and is called the half-breadth plan. All the plans are on the same scale. The straight lines on the deck are the curved lines of the sheer plan, or the tops of the several vertical planes. This plan reversed would show the other side of the vessel.