In an arched bridge, the higher it rises in proportion to the width of the arch, the easier is its construction, and the less is the stress upon its parts; moreover, any inaccuracy in design or in building is likely to be less harmful. We are not surprised, therefore, that Edwards, in his third attempt, decided upon that form. One of the widest arches in the world is that of the famous Grosvenor Bridge at Chester. It has a span of 200 feet, with a rise of 42 feet. An arch, however, in the Washington Aqueduct extends to 220 feet span, while the central span in the Southwark Bridge, designed by Rennie, is 240 feet. This last, however, is of cast-iron. The principle of the arch, however, does not appear first in the history of bridge building. Bridges are as old as mankind; that is, no one knows when first men began to cross streams and chasms by placing the trunk of a tree from one side to the other, and thus bridging the gulf. Then, possibly, the next step was to build up a pile of stones in the centre of the stream—taking the stones there by coracle or canoe—and placing a tree trunk from the side to the central heap. Yet another development would most likely be a simple cantilever bridge—though these early builders would not have known that Frenchified word. But they knew that after embedding a tree trunk firmly on each side of the bank so that a considerable portion should project over the stream, they could place a third log from one end to the other, and thus get a bridge much longer than when made of one tree trunk alone. 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. [Illustration: 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.