The bridge was a pretty good one, except for a slight unsteadiness between the center and either end.  When Uncle Ed saw it he showed us at once where the trouble lay.  Our intermediate cross beams were hung from the center of the spars, and consequently made them bend, because the strain came across their length, while at the center of the bridge there was no chance for the spars to bend, because the strain was exerted along their length, that is, it tended merely to push the ends of the spars deeper into the banks.  To remedy the trouble he proposed propping up the center of each spar with a brace running from the center crosspiece.  The dotted lines in Fig. 100 show how these braces were applied.  They made the floor perfectly solid throughout, and gave the bridge a much better appearance.  Uncle Ed told us that the structure might be called a “king rod truss,” except that in place of rods we had used wires.

The King Post Bridge.

The other bridge on the Schreiner property was built in the following summer, just before we started on our second expedition to Willow Clump Island.  It spanned the brook at the gorge, and was therefore a more difficult engineering feat.  Mr. Schreiner himself asked us to build it, and we felt greatly honored by the request.  A search was made in the Van Syckel library for a suitable type.  At last we found one that seemed properly suited to the requirements.  It was called a “king post truss,” and was very similar to the king rod bridge.  While the design of the bridge was simple, yet it required some ingenuity to put it together.  In setting up the other bridge the scow had been anchored in the center of the stream and used as a working platform, from which it had been an easy matter to put the various parts together.  In this case our scow was obviously of no use, so we laid a couple of long logs across the chasm, and a few slats were nailed across them to provide a temporary bridge or working platform.  The platform sagged considerably at the center, because the span was fully eighteen feet; but the logs were large, and we knew they were strong enough to support our weight.  However, as an extra precaution, we tied the ends to stakes driven in the ground, so that they could not possibly slip off the banks.

[Illustration:  Fig. 101.  The King Post Frame.]

[Illustration:  Fig. 102.  The King Posts Set in Position.]

[Illustration:  Fig. 103.  The Permanent Cross Beam Made Fast.]

First we set about constructing the king posts, which were made as shown in Fig. 101.  Two stout posts 7 feet long were connected at the top by a tie stick, which spaced them 4 feet apart.  To make a secure fastening they were notched together and strengthened with diagonal braces.  Each king post was notched on opposite sides, at about thirty inches from the top.  A temporary tie piece was also nailed across the lower ends of the king posts.  The frame thus formed was set up at the center of the span and temporarily