CHAPTER XXV.
THE CANTILEVER BRIDGE.
There is one more piece of work done by our society which yet remains to be described, and that is the cantilever bridge. This we all voted to be the greatest of our achievements on the island. To be sure, it was Uncle Ed’s design, but I think we justly deserve credit for the masterful way in which it was erected. In our search for types of bridges before building the king post bridge, we came across a simple cantilever bridge that didn’t look very difficult to construct. To be sure, none of us knew a thing about stresses and strains, and ingenious though we were, Bill realized that the task of designing a cantilever bridge was far beyond him. Nevertheless, we were sure we could build one if only we had a good set of plans. A letter was therefore mailed to Uncle Ed, asking him for the required details. The answer came promptly from Western Australia, asking us to send him the exact width of the water we wished to span, the depth of the water, the distance from the top of one bank to the top of the other, and the exact height of the banks above water level. We decided we would build the bridge across the mouth of the lagoon. The distance here between the two banks measured a little over 60 feet. The banks were very precipitous, and rose 13-1/2 feet above the level of the water. All these details, together with soundings of the bottom, all the way across, were sent to Uncle Ed, and on the day after our railway was completed quite a bulky package was received in answer. It contained complete directions for building the bridge of wooden frames, which were so designed that they needed merely to be hooked together to form the bridge, though to make the structure perfectly safe Uncle Ed cautioned us to tie the frames together wherever they met.
I am half afraid to tell my readers how to build this bridge, as it required the utmost care, and had to be built just so to avoid disaster. Bridge building is a serious business, and I would not advise any one to attempt building this, of all bridges, who does not propose to follow instructions implicitly. Uncle Ed told us that if we built it properly, and with sound timbers, we would find the bridge strong enough to support a dozen boys, but he warned us not to crowd more than that number on it.
Frames for the Cantilever Bridge.
Fig. 305. A Frame (make four).
The frames with which the cantilever bridge was built were made of saplings from 3 to 4 inches in diameter. We procured them from Mr. Schreiner’s lands up the river. In making the frames the sticks were fastened together with 1/2-inch bolts 6 inches long. It was quite a strain on our pocketbooks to buy these bolts, but Uncle Ed had written that nails or spikes would be useless to stand the strains of so large a bridge, and that if we
Fig. 306. B Frame (make four). could not get any bolts we had better give up the idea of building a cantilever bridge. To make sure that we made no mistakes, Uncle Ed had made a drawing of each different size of frame we would need, designating each with a different letter, and then these same letters were marked on a general view of the bridge, so that we would know exactly where the frames belonged. These drawings are reproduced here in Figs. 305 to 316 and 318. We had to