1848.] Construction of Iron Tension Bridges, 427 



stmcted resultant curve of a bridge of 1 60 feet span as designed, with 

 the several forces and angles delineated, and the subjoined table shows 

 the forces from which each link has been obtained, their magnitude 

 and direction ; it will be obvious that the horizontal tension of each 

 portion of platform supported by an oblique rod will be communicated 

 through the medium of the side longitudinal beams from the standard 

 to the centre, so that the tension on one half the bridge is counteracted 

 by that on the opposite half; this amount of tension in a loaded bridge 

 of large span is very great, (600 tons in a span of 500 feet, and 24 

 feet wide) being the sum of all the horizontal tensions A-f-B-j-C-r-D-J-E, 

 &c, and as the ends of these side beams are securely built into the 

 standard masonry, the swaying of the structure from side to side, or 

 undulation vertically under the influences of storms, or other ordinary 

 destructive causes, (excepting to a very slight extent) is prevented. At 

 the proof trial of the Bailee Khal bridge, 250 feet span, after its recon- 

 struction on the Resultant principle, the transit of a large elephant, and 

 24 pounder siege gun (See Fig. 17. also Table next page) with all its ap- 

 purtenances, caused no sensible vibration, or visible depression, whilst 

 at the conclusion of the ceremony the entire platform was covered with a 

 dense crowd of villagers, who, on the departure of the Governor and suite 

 came to witness the opening, and congregated as far as they were able to 

 one side of the bridge, thus giving fair proof of the stability and rigidity 

 of the structure. 



35. If therefore, as demonstrated by the Rev. Mr. Pratt, the 

 quantity of iron calculated to resist a certain dead weight, be the same 

 for bridges of equal span and width, and of equal strength, whether 

 the metal be distributed, as in the uniform system, or as in the " Re- 

 sultant," it surely is no small advantage in favour of the latter, that, by 

 construction, it is defended from the severe trials to which all bridges* 

 even when unloaded, are exposed, from the momentum which a compa- 

 ratively light body obtains when put in motion. 



36. The extra aid usually applied to suspension bridges on the 

 uniform system for the purpose of stiffening them, has been found 

 absolutely necessary, and duly commented on in paragraphs four and 

 five, and whilst such means are almost indispensable in the old system, 

 to compensate for vicious construction ; in the resultant system they 

 form an essential part of the principle ; and considering the results of 

 the experiments on a full-sized scale, (vide end of this memoir) the 



3 L 



