THE FOUTH BEIDGE. 433 



is the additional fact that the live load of a passing train will be quite 

 insigniflcant as compared with the dead load of the structure. The 

 heaviest train traversing the bridge will not deflect the 1,700-feet girder 

 more than 4 inches, which is considerably less th&n the deflection of the 

 460-feet span of Saltash Bridge under its test load, whilst a wind pressm-e, 

 equivalent to 30 lbs. per square foot over the entire 1,700 feet, would bend 

 the bridge laterally less than 9 inches. 



About 42,000 tons of steel will be used in the superstructure of the 

 main spans, and 3,000 tons of wrought iron in that of the viaduct ap- 

 proach. The total quantity of masonry in the piers and foundations will 

 be about 125,000 cubic yards, and the estimated cost of the entire work, 

 upon the basis of the prices at which the original suspension bridge was 

 contracted for is about 1,500,000Z. Owing to the varying price of steel, 

 and to the magnitude and novelty of the undertaking, this estimate must 

 be taken as approximate only, as a contract has not yet been concluded 

 for the works. 



It will be gathered from the preceding necessarily brief and incomplete 

 description of the proposed Forth Bridge that no novel or untried elements 

 enter into the design. In principle, a continuous girder is as old a type 

 of construction as an arch or a suspension bridge ; and probably, in pre- 

 historic times, streams were crossed by the aid of a couple of overhanging 

 branches and an intermediate central portion, constituting a structure 

 rude in appearance but identical in principle with the girder bridge. 

 The merit of the design, if any, will be found, not in the novelty of the 

 principles underlying it, but in the resolute application of well-tested 

 mechanical laws and experimental results to the somewhat diflicult pro- 

 blem offered by the construction of so large a bridge across so exposed an 

 estuary as the Firth of Forth 



1882. p F 



