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.

Erecting the Towers.

[Illustration:  Fig. 315.  K Frame (make one).]

[Illustration:  Fig. 316.  L Frame (make two).]

We built the complete set of frames before attempting to erect the bridge.  Then we began by building the towers.  Two A frames were set on end and spaced 4 feet apart at the top and 5 feet apart at the bottom, measuring not from the inner but from the outer edges of the frames.  In this position they were connected by short spars, notched in place.  The notches for these connecting spars will be seen in Fig. 305 on the main or vertical timbers of frame A, just below the upper and middle cross sticks and above the lower cross sticks.  The upper connecting spars were wedged tightly under the cross sticks, and served as an additional support for them.  Diagonal braces were nailed from one frame to the other, as illustrated in Fig. 318.  The towers were built on opposite banks, at the mouth of the lagoon, and when completed we lowered them carefully down the banks into the water.  According to directions they were to be set just 30 feet apart, measuring from the center of one tower to the center of the other.  The water was quite shallow where the towers rested, but the bottom was pretty firm.  Holes were dug in the bottom for the legs of the tower to set into, and then large stones were piled around each leg to provide a firm foundation for the towers.

[Illustration:  Fig. 317.  Notching the Sticks Together.]

Setting Up the Frames.

[Illustration:  Fig. 318.  View of Part of the Bridge, with Letters Indicating the Various Frames.]

A B frame was now hauled out to one of the towers and lifted by its narrower end, with fall and tackle, until its lower tie piece rested on the projecting ends of the center crosspieces of the tower.  The upper end of the frame was held against the top of the tower, while a C frame was hooked over the upper ends of the tower legs; then frame B was allowed to swing outward until its smaller end locked with the outer end of frame C. It will be observed in Fig. 306 that the upper crosspiece or tie piece of frame B was fastened to one side of the vertical sticks and the lower tie piece to the other side.  This was done purposely, so that when the frame was set in position the bottom tie piece would be on the lower side of the frame and the top piece would lie on the upper side, as shown in Fig 318, or, better still, in Fig. 319.  The rest of the frames were all arranged to be set in place with their tie pieces on the lower