The Scientific American Boy Part 23

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.

[Ill.u.s.tration: Fig. 305. A Frame (make four).]

[Ill.u.s.tration: Fig. 306. B Frame (make four).]

[Ill.u.s.tration: Fig. 307. C Frame (make four).]

[Ill.u.s.tration: Fig. 308. D 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 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 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 ill.u.s.trated 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 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.

[Ill.u.s.tration: Fig. 309. E Frame (make four).]

[Ill.u.s.tration: Fig. 310. F Frame (make two).]

[Ill.u.s.tration: Fig. 311. G Frame (make two).]

[Ill.u.s.tration: Fig. 312. H Frame (make one).]

[Ill.u.s.tration: Fig. 313. I Frame (make one).]

[Ill.u.s.tration: Fig. 314. J Frame (make one).]

[Ill.u.s.tration: Fis.h.i.+ng off the Cantilever Bridge.]

[Ill.u.s.tration: The Cantilever Bridge in Reddy's Back Yard.]

ERECTING THE TOWERS.

[Ill.u.s.tration: Fig. 315. K Frame (make one).]

[Ill.u.s.tration: Fig. 316. L Frame (make two).]

[Ill.u.s.tration: Fig. 317. Notching the Sticks Together.]

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 s.p.a.ced 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 ill.u.s.trated 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.

SETTING UP THE FRAMES.

[Ill.u.s.tration: 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 side, or facing the towers, as will be clearly understood by examining the ill.u.s.trations. As soon as the _B_ and _C_ frames were set up on one side of the tower, another pair of _B_ and _C_ frames was set up on the other side of the same tower. A cantilever bridge must always be built out on both sides of the tower at the same time, otherwise it will be overbalanced on one side and topple over. After the _B_ and _C_ frames were in place we took two _D_ frames, with oppositely cut ends, and rested their tie sticks on the top of the tower, just under the ends of the _C_ frames. The ends of the two _D_ frames overlapped at the center of the tower, and, as one was cut away at the outer side and the other at the inner side, they fitted neatly together and were fastened with bolts. The _D_ frames were supported near their outer ends with _E_ frames, which rested on the _B_ and _C_ frames. Fig. 319 shows an _E_ frame set in position on the landward side of the tower, while two of the boys are climbing out on the opposite _B_ and _C_ frames preparatory to setting up the other _E_ frame. A cross stick was now bolted to each _D_ frame, just beyond the upper ends of the _E_ sticks.

This done, the frame _F_ was hooked in between the ends of _B_ and _C_, at the sh.o.r.eward side of the tower, and its outer ends were supported by frame _G_, which was hooked over frame _D_ and the upper ends of frame _E_. The frame _L_ was then rested on the ends of frame _F_ and _G_, and supported the sh.o.r.e end of frame _D_. A stick nailed across frame _D_ on each side of the upper ends of frame _L_ served to hold the latter in place.

BINDING AND ANCHORING THE STRUCTURE.

[Ill.u.s.tration: Fig. 319. Preparing to Put an E Frame in Place.]

As the different frames were coupled together, we bound the overlapping ends with soft iron wire. The place where frames _B_, _C_, _E_ and _F_ came together was quite a vital point, and we took pains to make the wire binding at this place doubly strong. As soon as the _L_ frame was in place we anch.o.r.ed the bridge to sh.o.r.e by running wires from the ends of the _D_ frame and the ends of the _G_ frame to stakes driven into the banks. The frames on the second tower were now similarly erected and anch.o.r.ed, after which we were ready to put in the center panels of the bridge.

THE CENTER PANELS OF THE BRIDGE.

First, the frame _H_ was wedged into place and thoroughly fastened by a liberal winding of wire. Next the frames _I_ and _J_ were set in place, and in order to do this we had to remove the upper tie pieces of these frames. Then one frame was hooked in the other, and the two were carried out on the scow under the center of the bridge. Ropes were tied to the ends of the two frames, and they were lifted together, like a wide _V_, to the position shown in Fig. 318, after which the tie pieces were bolted on again, resting against the ends of the _E_ frames. As an additional security, two sticks were bolted to the under side of the frame _H_, one at each side of the _I_ and _J_ frames. The bridge was then completed by wedging the frame _K_ under the ends of the _D_ frames, and also placing a stick across each tower under the joints of the _D_ frames. We planned to run our gravity railway across this bridge, moving our platform and trestle to the opposite bank; so instead of flooring our bridge with slabs, we fastened ties across at intervals of 15 or 18 inches. These ties were sticks 3 inches in diameter, which were secured to the _D_ frames.

A SERIOUS INTERRUPTION.

[Ill.u.s.tration: Fig. 320. A General View of our Cantilever Bridge.]

We were just preparing to lay the tracks across the bridge when we met with a serious interruption. Mr. Halliday had told us that a few days before our arrival that summer Mr. Smith, the owner of the island, and another man had paid a visit to the place. Jim Halliday himself had rowed them over, and learned from their conversation that Mr. Smith was trying to sell the island, and that the stranger, a Mr. Gill, was a prospective purchaser. All summer long we had been dreading the return of this customer, though, as time pa.s.sed without his putting in an appearance, we almost forgot the incident. But now, at the end of August, just as we had about completed our cantilever bridge, who should arrive but this very man Gill and three other men with a large tent and camping outfit. It was a sorrowful crowd of boys that watched the wagon with their belongings ford the shallow water over to our island. We felt that the island was ours by right of discovery and occupation, but we were powerless to force our claims. And what if they did not insist on our leaving the island? It would not be the same place with strangers around to meddle with our things.

DISPOSSESSED.

But the new owner of the island was even more of a boor than we had antic.i.p.ated. As soon as he landed he wanted to know what we were doing on his property, and peremptorily ordered us off. Bill answered that we were camping there, and politely asked if we couldn't stay out the summer. But Gill would not listen to the idea. We must get off the island that very day or he would see to it that we did.

I tell you it made us boil. We were just itching to give the pompous little man the sound thras.h.i.+ng he deserved, but knew that we would stand a very small show against his three powerful companions. At any rate, we were determined not to leave at once. Instead, we repaired to Kite Island, taking our belongings with us. Then we cut away the suspension, spar and pontoon bridges, so that we would not be annoyed by any of the Gill crowd. We were resolved that they should not benefit by any of the things we had built.

At the dead of night we paddled back to Willow Clump Island, crept past the slumbering intruders and waded out to the old water wheel. After a good deal of exertion we managed to dislodge the smaller tower, letting the wheel drop into the river and float away. Then we made for the cantilever bridge. It didn't take us very long to cut away the wire bindings, unhook the frames and drop them into the lagoon. But the task was quite a perilous one, as the night was pitch black. Finally, nothing remained of the bridge but the two towers, which were left as monuments to mark the spot where our last piece of engineering on the island was done.

[Ill.u.s.tration: Building the Cantilever Bridge over Cedar Brook.]

FAREWELL TO WILLOW CLUMP ISLAND.

We spent several days on Kite Island, knowing that we were safe from intrusion, because the Gill crowd had but one boat, and that was on the Jersey side of the island. We felt confident that they would not take the trouble of wading around Point Lookout with their boats, as we had done; nevertheless, to prevent a surprise, we kept a sentry posted on the Lake Placid side of the island and gathered a pile of stones for ammunition. But our fun was spoiled, and we finally decided to break camp and bid farewell forever to Willow Clump Island and its vicinity.

Our goods were ferried over to Jim Halliday's farm, where we were given shelter. The windmill, as I have already stated, was sold to a farmer at Lumberville, and we were kept busy for several days carting it over and setting it up in place. When everything had been done we stole back to Kite Island and set fire to the log cabin. The next day Mr. Schreiner took us home in a couple of his wagons. Thus ended our "investigation, exploration and exploitation of Willow Clump Island." The work of two summers was practically all destroyed in a few days.