Polytechnic Association Proceedings. 829 



The requisite horizontal stiflfness was obtained in the following 

 way: " The carriage way was formed of four thicknesses of 

 Memel planks, the two lower layers, each two inches thick, were 

 placed diagonally with the transvere beams crossing each other so 

 as to form a reticulated floor, abutted against the longitudial beams. 

 They were firmly spiked to the beams and to each other at all the 

 intersections, and upon them was laid and spiked a longitudinal 

 layer of planking two inches thick. Each layer was closely jointed 

 and caulked, and the upper one was laid in a mixture of pitch and 

 tar; a composition of fine gravel and sand cemented with boiled 

 gas-tar was laid over the whole, to the thickness of one inch, form- 

 ing the road track." • 



The absence of Brunei's counter chains and of Brown's counter 

 stays, and other unnecessary and even dangerous furniture, is to be 

 regarded as highly commendable. 



"The whole formed a compact mass of braced wood work, the 

 diagonal planking giving the horizontal stiifness, and the two 

 trussed frames insuring the vertical rigidity." 



It is interesting to know the weights of this structure, as import- 

 ant concerning the art of building suspension bridges with stiffened 

 floors. 



The weight of the roadwork was 131 tons. 



" of cast and wrought iron 37 tons. 



" of suspension rods 21 tons. 



" of fencing (wrought iron) 9 tons. 



" gravel concrete 30 tons. 



Total 228 tons. 



Per running foot 0.53 tons, 



or forty-seven and a half pounds per superficial square foot, for the 

 entire roadway. Subtracting the gravel concrete, it is forty-one 

 pounds. Before the bridge was stifiened, the weight was forty- 

 four pounds per square foot. 



The bridge has borne without injury heavy gales, as could be 

 foreseen, and the stiffness has given confidence in the strength to 

 all who have examined it. 



The English engineers agreed with Rendel, and the engineer 

 Vignoles, eulogized this excellent communication for the practical 

 conclusions which it contained. He found it not difficult to fore- 

 see, that by carrjing out the system of adopting well trussed fram- 

 ings to the platforms of suspension bridges, sufficient rigidity would 

 have been obtained for locomotive engines and carriages on rail- 



