This principle, known so long ago, was used and immensely developed in the construction of the famous Forth Bridge, one of the most remarkable structures of the nineteenth century. This cantilever principle is very important in bridge building, and it is said that there exists an ancient bridge on this principle across the Sutlej in India with a span of 200 feet.
THE POST BRIDGE, DARTMOOR.
(An example of an early bridge, of “slab” construction.)
A further variety of early bridges was the “slab” bridge, consisting of slabs of granite placed from side to side, or from the sides of the bank to heaps of stones piled up in the stream. A good example of such a bridge may be seen at “Post Bridge” over the Dart on Dartmoor. Ages ago this bridge was built, and as we study it and compare it with the modern structure not far distant, we wonder how the ancient Britons—if those sturdy individuals are really responsible for it—could raise and place those huge slabs of stone without engineering apparatus. Probably it was done with levers and rollers, and there must have been many shoulders to the wheel in the process. Certainly they had plenty of granite at hand on wild Dartmoor.
But passing by all these early forms of bridges—which it will be noticed are built of a few large pieces of material—it was left to the Romans, at all events in Europe, to largely adopt the arch as a principle of construction.
Now, here we are dealing with an altogether different principle. The arch is made up of a number of comparatively small pieces of material bound together by mortar, or cement, or even clamps, and by the power of gravitation.
We doubt if that idea is realised by half the people using the multitudinous arches abounding to-day; yet it is true. Or to put it in another way, the various parts are arranged so that they keep up each other by pressure.
If you take two cards, or bricks, or slabs of stone and lean them together at the top, while the other ends may be far apart, you will find they will bear a certain amount of weight. Here you have the principle of the arch in its simplest form; and it may be that out of that primitive performance the arch has grown. This kind of triangular arch is to be met with in ancient structures in Great Britain. The flanks or haunches of an arch are its sides, from the first stone to the keystone; and the crown is its highest part; while the central wedge-shaped piece of stone or brick is called the keystone.
The stones or bricks are cemented together when being built over a framework of timber, called the centering, and when the keystone is placed and the arch is complete it ought to remain firm.