Reduced to its simplest expression, and as it has actually been used for unknown periods in Asia and in South America, a suspension-bridge consists of two parallel ropes swung from side to side of a ravine, and carrying the platform over which the passenger walks. As the span increases, so that the dip makes the ropes impracticable, the land ends of the ropes are hoisted some distance above the roadway which they carry. If nothing can be found there strong enough to hold them, they are simply passed over, say, forked trees, and the ends made fast to other trees or held down with stones. This is the essential construction of the Brooklyn Bridge. The ropes become four cables sixteen inches thick, of 5541 steel wires; the forked tree becomes a tower 276 feet high, and 8260 square feet in area at the base; the bowlder to hold down the end of the rope becomes a mass of masonry of 60,000 tons’ weight; the shaky platform becomes a great street, 85 feet wide, of five firm roadways. But the man who first carried his rope over the forked tree was the inventor of the arrangement which, developed through all the refinements of modern mechanics, forms the groundwork of the Brooklyn Bridge. This statement of the germinal idea of a chain-bridge will, perhaps, give a clearer notion of the functions of the several parts of the Brooklyn Bridge than a consideration of the complicated structure in its ultimate evolution, in which these functions are partly lost sight of. But if the structure had been architecturally designed, these things would have been emphasized at every point and in every way. The function of the great “towers,” so called, being merely to hold up the cables, it is plain that three isolated piers would have performed that [Illustration: BRIDGE AT MINNEAPOLIS. Thomas M. Griffith, Engineer.]