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POPULAR SCIENCE REVIEW. 
To break one of the girders of the Britannia Bridge would 
require about 5200 tons distributed uniformly over the 460 
feet span, including the weight of the tube. The weight of the 
tube is 1550 tons, and the weight of a train of the most 
heavily laden trucks, extending over the entire span, would be 
about 570 tons. If we deduct the weight of the bridge from 
the breaking weight we have a surplus strength of 3650 tons 
to resist the rolling loads, or the proportion of six and a half 
to one, nearly. Or according to another method of calculating 
the strength, for the authorities differ at present as to the 
proper plan, 1550 + 570 ~ 2120 tons, which must be con- 
sidered as the maximum load which can be brought on the 
bridge. The strength will then be nearly two and a half times 
the maximum load.* The deflection of one of the Conway 
girders from its own weight was nine inches in the middle ; 
that of the larger Britannia girders twelve inches. 
IV. The Construction of the Tubes . — The tubes for the Conway 
Bridge were constructed on a platform on the beach of the river, 
420 feet long by 40 wide, workshops containing shearing, 
punching, and riveting machines, noth furnaces and forges, 
being erected on the land close by. The plates as they were 
wanted were straightened by a hideously noisy process of ham- 
mering on massive cast-iron bed-plates, or by the quieter pro- 
cess of passing- through rolls. They were then sheared to the 
exact size, and the positions of the rivet holes marked ready for 
punching. The ordinary punching machines have one, two, or 
three punches which rise and descend with a slow reciprocating- 
motion of about four strokes a minute, and they have dies of 
the same size beneath. The workman adjusts the plate with 
great dexterity between the punch and die, so that at each 
descent of the former a hole is made in the required position. 
This is effected with the greatest ease, although forty tons 
pressure is necessary to punch a hole of ordinary size in a 
three-quarter inch plate. A very remarkable self-acting punch- 
ing- machine, by Mr. Roberts, was partially employed for the 
Britannia Bridge. It worked on the principle of the Jacquard 
loom, and punched any pattern of holes across the whole width 
of the plate without previous marking, the plate being carried 
forward on a sliding- table. The importance of the operation 
may be judged from the fact that, according to Mr. Clark’s 
calculations, there are seven millions of holes in the Britannia 
Bridge alone. The plate when punched was removed to the 
erecting platform, and carried by means of a travelling crane, 
moveable, over the whole extent of the stage, to the position 
it was to occupy in the girder. 
* The effect of the continuity of the girders in strengthening the bridge 
has, however, been neglected in these calculations. 
