902 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



tensile strain, Avhen cooled in the ordinary way, from 90 to 120,000 

 pounds to the square inch, but when cooled in oil, the resistance 

 to the tensile strain rose from 12 to 5G per cent., according to the 

 quality of the bar tested, the average being 40 per cent. Taking 

 a judicious mean of his experiments upon the latter mode of cool- 

 ino', gave a result of from 53 to 60 tons per square inch of section. 

 I have, therefore, assumed in my computations 15 tons to the 

 square inch as a safe unit, little less than one-fourth the breaking 

 strain of the best quality. For a railway bridge I would allow a 

 fn-cater maigin for safety, 'o absorb the incidental shocks of heavy 

 machinery, but the case before us, when at best the weight of the 

 variable load is so disproportionate to the weight of the perma- 

 nent load, that I believe the above limit to be ample provision. 

 When compressive forces arc called into action, the safe limit 

 assumed for forged' steel is 12 tons to the square inch, the ratio of 

 steel to tension and compression being taken at the same ratio 

 possessed by Avrought iron. 



Description of the Proposed Bridge. 

 I have called this system in my classification the trussed suspen- 

 sion system (Fig. 6). Kecognizing the great strength in the sus- 

 pension bridge, I have adopted the principle to carry the load, 

 but do away with the high towers and anchor masonry entirely. 

 This is done, as you notice, by causing the chains to react against 

 a horizontal member, in the manner shown, which member is kept 

 in place b}'' an arrangement of diagonals and verticals. It is, in a 

 few words, a trussed fiu-ncular polygon, to which class all trussed 

 o-irders belong, of the trapezoidal form, used so much in all depart- 

 ments of construction. A trussed pui-lin, by means of the under 

 struts and suspension rods, call into play the same forces, only 

 in a modified degree. The elder Brunei constructed an enormous 

 trapezoidal truss over the Wye, at Chepstow, the span being 

 thre hundred feet. There are but tico under struts, supporting 

 the horizontal member, which was composed of boiler-plate § 

 iron, and made tabular, with a diameter of nine feet. The depth 

 of truss is, I l)elieve, 50 feet, the railway passing under the tube 

 and through the struts. I make these remarks to show you that 

 I am attempting nothing new, only putting on a large scale Mhat 

 you every day see before you. The diagram connected herewith 

 represents one span of G50 feet, there being two in the design, 

 supported by a central pier. You will notice that the design is 



