246 REPORT—1857. 
At the time they were designed, the popular objections to suspension 
bridges were much greater than at present, and no example existed of a rail- 
way suspension bridge. 
An engineer might then have been as little justified under such circum- 
stances in adopting a suspension bridge for railway traffic, as he would now 
be in error in disregarding the experience which has since been- obtained. 
It is still, however, asserted, but without any assigned reason, that suspen~ 
sion bridges are not adapted for trains at speed; my own view on this point, 
from large experience iv railway construction, from observing the effect pro- 
duced on bridges crossed by contractors’ waggons drawn by horses, and by 
experiments made on trains at speed with the Iron Commissioners, is, that 
road traffic gives as severe trial by troops marching in step, by herds of 
cattle, or by cavalry trotting or galloping, as the heaviest trains at full speed 
on railways. 
This is not, however, the subject I now submit for discussion ; the first 
step in the inquiry is the simple mechanical problem of the strength and 
deflection with stationary loads, on which no doubt should exist ; and when 
it is remembered that the extension of the railway system is much governed 
by the cost of construction, of which the crossing of valleys and rivers forms 
so considerable an item, that in some cases a single bridge costs as much 
as 75 or 100 miles of line, I hope the inquiry will be deemed of sufficient 
importance by the Association to elicit a full investigation and discussion. 
APPENDIX. 
ae 
Estimate of Deflection of the Londonderry Girder, from experiments on the 
Boyne Viaduct. 
The centre opening is 264 feet. Weight of girder 300 tons. 540 tons all 
over produces a deflection of 1:9 inch. 
The deflection, if of the length of the Londonderry Bridge, would have 
been 264° : 440% :: 1:9-: 8°79 inches. 
To ascertain the deflection, if of the same depth as the Londonderry 
Bridge, we have 16°5° : 22°5* : : 8°79 : 22-289 inches. 
This assumes a weight per foot forward equal to the Boyne Viaduct. The 
Boyne Viaduct, if of the same length as the Londonderry Bridge, would 
weigh 512 tons. 
The following will therefore be the deflection, if of the same weight as the 
Derry Bridge :—150 : 512 :: 22-289 : 76°078 inches, which is the deflection 
with 540 tons all over. 200 tons all over will therefore be 28°17; 100 tons 
in the middle, 23°53 inches. 
Estimate of the Deflection from Experiments on the Newark Dyke Bridge. 
Span, 240 feet ; weight of girder, 2444 tons; deflection with 240 tons all 
over, 2°75 inches. As 240% : 440° :: 2°75 : 17 inches. 
The depth of the Newark Dyke being the same as the proposed London- 
derry Bridge, 17 inches will indicate the deflection, if it was equal in weight 
to the Newark Dyke Bridge; but the weight, if of the same length, being 
4.50 tons, we have,—150 : 450 :: 17 : 51 inches, the deflection with 240 
tons all over. With 200 tons all over, 42°5 inches. 
With 100 tons in the middle it will therefore be 34 inches. 
Estimate of the Deflection from Experiments on the Britannia Tube. 
The Britannia Tube weighs 1600 tons, and deflects with 200 tons all over 
125 inch. 
