1838.] Ojx the Construction of Suspension Bridges^ 



121 



" first direction, passes round one x^ulley to preserve the parallelism, 

 " and then on to another about sixteen inches distant, from which it 

 " again proceeds over the river and passes round a second pulley on 

 " that side, and finally returns to the side from which it parted, and is 

 made fast to a bolt in the rock. Thus it crosses the river four times* 

 ** Small cross pieces of w^ood are attached at intervals to these redu- 

 " plications of the band, and over them are placed the planks, parallel 

 " to the wires, which form the foot-way of the bridge (Plate 

 fig 5J. Two other bands of wire are carried across the river at a 

 " convenient height on each side of the bridge, to serve as hand-rails ; 

 " they are connected by descending wires to the external bands of the 

 " bridge : and, to prevent every lateral motion the bridge is made fast 

 " at the middle to some large stones in the bed of the river. 



" This bridge though of a structure so light as to occasion fear on 

 " first time of going on to ii, is yet so steady and strong that no sensi- 

 " ble bending or vibration is perceived in passing over it. It is two feet 

 " broad, and fifty-five feet long. The weight of iron wire used in its 

 *' construction was about twenty-four pounds, and the expense of the 

 " whole of the materials amounted to little more than 35 francs. 

 " The expense of labour is estimated at about 15 francs, so that 50 

 " francs according to this account would pay for the whole." 



My calculation for the above bridge is as follows : — 

 Wire ^Vth inch diameter has a sectional 



area of , 0.0019635 inches. 



This multiplied by the number of wires 32 



39270 

 58905 



.0628320 sqv. in. 

 38 



5026560 

 188496 



2.3876160 



Allowing for depreciation of strength by 

 straining, and taking 38 tons pr. square 

 inch Tons . . 



We have neraly 2| tons for the power of the wire employed, but as 

 the planking for 110 square feet and the weight of the wire would make 

 up half a ton, this bridge could safely bear 1 ton, or thirty men, era 

 loaded palankeen and 12 bearers, were it wide enough. 



