900 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



what the elements are, from Avhich you may draw what coiiclnsions 

 you please. In the iirst place our 1:)ridge must be 140 feet high, 

 to the under side of the roadway. With span as above assumed, 

 the versed line of cables would be a minimum at ^\- the chord line, 

 or say 90 feet. This gives a total height of 230 feet, or only 50 

 feet lower than Trinity church steeple. The angle formed with 

 the vertical would be 75° 04', necessitating a distance into the city 

 of 876 feet before striking the surface of the ground, which would 

 be the commencement of the anchorage, unless the masonry was 

 carried up a considerable distance above the surfoce, when the 

 anchorage would move proportionately towards the river. Such 

 a bridge would weigh, with its maximum load, about 10,000 tons, 

 causing a horizontal strain in the cables of over 18,000 tons, 1)ear- 

 ing upon the summits of the towers with a crushing force of 5,000 

 tons, and endeavoring to pull up its anchorage with a force of over 

 19.000 tons. When you reflect upon the great mass of this anchor 

 masonry, the magnitude of the city termini can better be under- 

 stood, and must be one of the governing points in the selection of 

 this system of bridging for the East river, irrespective of the ques- 

 tion of cost. It is in view of these facts, that I present for your 

 consideration the following system, as illustrated in the subjoined 

 diagrams. 



Materi.u. to be Used. 

 Before doing so, however, I desire to say a few Avords about 

 the material that I propose to employ, and also the reasons that 

 induced me to assume a high unit of safety. Could we have a 

 material possessing strength without weight, and in all other 

 respects applicable to bridge construction, the amount Avhicli 

 would be used for various spans, would be in proportion to the 

 squares of the Avidth of openings, and spans of intinite length 

 would be practicable. In a bridge, part of the work that it has 

 to do, is made up from the absolute weight of the material used 

 in its construction, and therefore the limit of space is reached in 

 all bridges, Avhen the dead weight or permanent load, would over- 

 task its powers of resistance, involving, of course, its destruction. 

 To double a span, and retain the strength of a bridge, would 

 quadruple its weight, so that its loads would increase twice as fast 

 as the increase of strength. Thus in the Britannia bridge of 460 

 feet span, with all the skill brought to bear in the distribution of 

 material, the strain is given as six tons to the square inch (London 



