Our model shall be a fifteen inch one. LAYING OUT MATERIALS is the next thing in order. First we want a sheet of brass for the bed plate, 1/16th of an inch thick, cut 4×14 inches, and be sure to cut the corners square. (See Figure 3.) Hammer this out flat, file it smooth and dress up, with emery cloth fastened upon a flat piece of wood. Next cut a square hole in it as at C, beginning half an inch from B, and making the opening 11 × 1-1/2 inches. Be careful to center this hole on the line A B, or your engine will be lopsided, and you must take the same care in setting the smoke stack, dome, etc. Now take Fig. 4. This represents one of the side frames. Cut these out now, thus: Drill holes at A B C for the axles to work in. Finish both sides the same way. Turn the bed plate upside down, fasten the frames on at a quarter of an inch from either side by small angle pieces (Fig. 5), or by soldering, which is easier done. Then solder a piece across each end, about half an inch deep, and the frame is ready for the wheels. These you can make if you have a lathe, but it would be better to buy your wheels ready made if you can, but if you can't do that, and have the lathe, turn your tires up to the form shown in Fig. 6. The small wheels should be about 2-1/2 inches in diameter, and the driving wheels, 4 inches. The rim, B, should project a little over 1/16th of an inch, and the rest of the edge should be beveled off rightly, as at A. The spokes should then be filed up smooth, drilling out the center hole for the axle before removing it from the lathe. Great care must be taken to turn both the driving wheels to exactly the same diameter, or one wheel would travel further in a revolution than the other, and as they ought both to be fixed rigidly on the crank shaft, the engine would never travel in a straight line, but would go round and round in a circle. Get some steel wire for the axles and fasten them to the wheels by soldering or by cutting a slot with a fine file in the center of the wheel, as at A, Fig. 7. Then file a small portion of the ends of the axle flat and drive in a brass wedge made by a piece of wire which will hold them together firmly. The crank shaft, or axle, must be hammered up to shape, making it hot occasionally in the gas flame while working it. The cranks should be at right angles to each other, and the throw of the crank half the distance of the cylinder stroke. For instance, say the cylinder being a 1-1/2 inch stroke, the distance between A B (Fig. 8) will be three-quarters of an inch, you must then ease the size of the crank at A to prevent the piston knocking the cylinder ends. [Illustration: Fig. 9.] The cylinders you had better buy ready made or have them made for you. Get a pair of oscillating cylinders of three-quarter inch bore and inch and a half stroke. These will drive your engines several miles an hour.