Mr. Seaward 07i Suspension Chain Bridges. 4-31 



is reduced one-half, while the load is uniformly distributed 

 over the bridge ; yet it is likely to happen that such an accu- 

 mulation of weight may take place at some one point, as would 

 produce a greater strain than could possibly occur in the 

 catenary curve ; because under such circumstances the whole 

 weight would have to be supported by one set of rods only ; 

 whereas in the catenary curve every part of the chain would 

 be called into action, and therefore the strain upon the latter 

 would be proportionably much less." 



In answer to this objection it is proper to remark, that the 

 greatest accumulation of weight that could possibly occur to 

 affect the suspending rods on the new plan, would most likely 

 be by two heavy road waggons going over the bridge at the 

 same time in contrary directions, and passing each other ; 

 it being presumed that, in such case, the whole weight of the 

 two waggons would be thrown on one set of rods only. 



Now the weight of two large road waggons, with their load- 

 ing included (as limited by act of parliament), is 13 tons; but 

 which may be stated at 15 tons, to compensate for any casual 

 over-weight : and the weight of every portion of the platform 

 ab, be, cd, &c. is about 7i tons, making together 22\ tons, 

 which, under these circumstances, may be thrown on one set 

 of the rods. But it has already been shown, that a weight of 

 15 tons passing perpendicularly would cause a strain of 7 tons 

 per square inch of sectional area on the diagonal rods ; of 

 course a weight of 22^ tons would increase the strain to 10^ 

 tons per square inch. Whence it would appear that circum- 

 stances might occur to throw a strain on the rods of ?,\ tons 

 per square inch more than what it was proposed they should 

 sustain : and thus the presumed merits of the plan would be 

 considerably diminished. 



Now admitting, for the sake of argument, that the above 

 conclusions are just, it is but fair to remark, with respect to 

 the catenary curve, that it is quite as likely this latter may be 

 affected by a strain of 15 tons per square inch, as that the 

 diagonal rods, according to the foregoing reasoning, may be 

 affected with a strain of 10^ tons per square inch. Therefore, 

 allowing the full force of the objection, it is still manifest that 

 the adv^antages are decidedly in favour of the new plan. 



But on examining the matter a little more closely, it will be 

 found that die objection rests upon two assumptions which 

 are quite inadmissible. In the first ])lace, it is assumed that 

 the whole weight of the two waggons must necessarily be 

 thrown upon one point of the bridge, and consequently sus- 

 tained by one set of rods only. But the weight of the two 

 waggons never can be so drawn into one point ; because the 



hinder 



