Notwithstanding the skill displayed in its construction, this bridge has, since the above account was written, been destroyed by a tremendous hurricane.
FIG. 147_b_.—_Living Model of the Cantilever Principle._ ]
The great Forth Bridge, now (December, 1889) approaching completion, is the first bridge on the cantilever and central girder principle that has been erected in Great Britain, and it has also the distinction of being by far the widest spanned bridge in all the world. We are told by the engineers of the bridge that the cantilever and girder principle is by no means new, for it has been adopted hundreds of years ago by comparatively rude tribes in the construction of timber bridges, to which it readily lends itself. Such bridges are described as having been erected by the natives of Hindoostan, Canada, Thibet, etc., even at remote periods. The principle of the cantilever and girder construction was well illustrated by Mr. Baker, one of the engineers of the bridge, at a lecture given by him at the Royal Institution, by means of what he termed “a living model,” of which (Fig. 147_b_) shows the general arrangement. Two men, seated on chairs, extend their arms and hold in their hands sticks, of which the other ends butt against the chairs. The central girder is represented by a shorter stick, suspended at _a_ and _b_. We have here the representation of two double cantilevers, the ropes at _c_ and _d_, connected with the weights, representing the anchorages of the landward arms of the cantilevers. When a weight is placed on _a b_, which was done in the “living model,” by a third man seating himself thereon, a tensile strain comes into action in the ropes and in the men’s arms, while the sticks abutting on the chairs have to resist a compressing force, and the weight of the whole is borne by the legs of the chairs, also under compression. Now let the reader imagine the men’s heads to be 360 feet above the ground, and about a third of a mile apart, while the distance between _a_ and _b_ is 350 feet, and he will have a rough but sufficiently clear idea, not only of the principle upon which the Forth Bridge is constructed, but also of the magnitude of one of its spans. To complete the comparison, Mr. Baker further invited his hearers to suppose that the pull upon each arm of the men is equal to 10,000 tons, and that the legs of each chair press on the ground with the weight of more than 100,000 tons.