You will need four pieces of timber, seven feet long and two inches thick by four inches wide, for the uprights or corner-posts (A and B, Fig. 131), unless the posts and rails of the back fence are on your side of the yard, as in Fig. 132. In this case you need only two seven-foot sticks and two short ones, to fit on the top rail of the fence. The tops of these short posts should be just seven feet from the ground. Nail them securely in place, about five feet apart, as in Fig. 132, and then see that the fence-boards, to which the posts are nailed, are secure. If they are not secure, climb over the fence and put in a few good wire nails, for if the fence is not strong your structure will be weak. Additional strength may be gained by making each of the uprights of two pieces of two-by-four, nailed together, thus making the posts four-by-four. [Illustration: FIG. 131.] [Illustration: FIG. 132.] It is a good plan to erect one of the posts directly over one of the fence-posts; this will add strength and stiffness to the structure. If you have any doubts about the ability of the fence to support the platform, erect two seven-foot posts, as in Fig. 131, and spike them to the top and bottom rail of the fence. Next take two pieces of two-by-four and notch them, as E and F are notched in Fig. 133. Nail F to the top of A and B, and E to the ends of C and D, the two seven-foot posts of two-by-four. Near the other ends of these last posts nail a cross-piece (G, Fig. 133), and then, to stiffen the frame, turn it over and nail on two diagonal pieces of lighter material. [Illustration: FIG. 133.] Erect this frame about five or six feet from the fence and secure it in place by the two diagonals, H and H (Fig. 133), which are nailed near the top of C and D, and “toe-nailed” to the bottom rail of the fence. [Illustration: FIG. 134.] From the bottom rail of the fence, and level with it, run the two J pieces of board to C and D, and nail them securely, as in Fig. 134; then nail on the two top side-pieces, K and K, and the framework of the starting platform is ready for its floor. Nail boards across the top, from E to F, and saw off the protruding ends, as in Fig. 134. [Illustration: FIGS. 135, 136, and 137.] The Track must be a “straight-away,” which means no curves to round, hence you must build it in the position which will give the longest run for your trouble. A Curved Track