From this model, "life-size" plans of the ship are made with chalk on the floor of a long, wide room, like a big garret, which is used especially for this purpose. It will not be necessary to enter into a technical description of these plans. There are three of them,--the sheer plan, the half-breadth plan, and the body plan. They show the position of the different planks to be used in the construction of the ship. To gain a rough idea of these plans, take a cucumber, decide which you will call the bottom and which the top, and cut it in the middle, lengthwise, from end to end. Look into its interior and fancy that it is covered with lines, both horizontal and vertical--and that will give you a very rough idea of the sheer plan. By laying the cucumber on its side and cutting it lengthwise, you will have a notion of the half-breadth plan. A division in the middle (cutting it in two parts, so that you can see the whole circumference) may suggest to you the body plan. This can not be made very clear, not even with drawings, because it is the most technical part of the work; but its object is apparent. From these three plans, taken from different points of view, the boat-builder can locate the position of every piece of plank in his vessel. So true is this that I understand it is possible to number the planks of a ship, and send them off to some distant country, where a ship-builder can construct the vessel without ever having seen the design. A great deal of calculation and figuring enters into this part of the work, but much of it has been made easy by the aid of a man (now dead, I believe) named Simpson, the author of what are called "Simpson's Rules." These rules are incorporated in small pocket handbooks which contain, in addition, a large number of tables, rules, and formulas pertaining to naval architecture. The most popular handbook of this character in England is said to be "Mackrow's Naval Architect and Ship-builders' Assistant," and in our country, "Haswell's Engineers' Pocket-book of Tables." These, however, are only aids in making calculations, and are very much like the interest tables you have probably seen, which save the trouble of going through the figuring in detail. There are a great many books which will be interesting and valuable to the young ship-builder. To give you some idea of their character, I copy the following from the table of contents of a recent standard work: "The displacement and buoyancy of ships;" "The oscillations of ships in still water;" "The oscillation of ships among waves;" "Methods of observing the rolling and pitching motions of ships;" "The structural strength of ships," etc. These titles may not at present indicate a very promising literary feast, but when the young boat-builder has mastered the rudiments of the technical part of the profession, he will read and reread such productions with as much pleasure as he now peruses the stories in ST. NICHOLAS.