Fig. 307. C Frame (make four). make four frames each, of the A, B, C and E sizes, two each of the F, G and L sizes and one each of the H, I, J and K sizes. Of the D frames two were made with the ends cut away on the outer half, as illustrated in Fig. 308, and two were cut away at the inner side, the reason for which will appear presently. When fastening the timbers together we cut notches in each stick, as shown in Fig. 317. The depth of each notch was just one-quarter the diameter of the stick; that is, the notch was 3/4 of an inch deep in a 3-inch stick and 1 inch deep in a 4-inch stick. Care was taken not to exceed this depth, for fear of weakening the sticks. In the case of frame D, the sticks were not notched or mortised together. It will be noticed that the measurements are given to the inner edges of the sticks in some cases, and to the outer edges in others. The reason for this, as Uncle Ed explained it, was because the
Fig. 308. D Frame (make four). thickness of our sticks would vary considerably, and it was important that many of the measurements be exact, otherwise the frames would not fit into each other as they should. Another thing to which he called our attention was the fact that frames A, B, E, F, H, K and L were stiffened with cross braces, while the rest were not. The braced frames, he wrote, were those which would be under a compression strain, while the others would be under tension; that is, when any weight was placed on the bridge it would push against the ends of the braced frames, trying to crush them, but would pull on the unbraced frames, trying to tear them apart. In fact, the bridge would have been just as strong had we used heavy iron wire in place of the unbraced frames, and the only reason Uncle Ed did not recommend our doing so was because we had no simple way of stretching the wire taut.
Fig. 309. E Frame (make four).
Fig. 310. F Frame (make two).