M r Stevenson’s Description of Bridges of Suspension. £53 
easion, professionally, to examine the strength of iron, as a 
material which enters largely and essentially into the construc- 
tion of bridges of suspension. He has also considered its laws 
of expansion, and other points bearing upon this important sub- 
ject,— but without entering at present upon these topics, he must 
observe, that if we were to allow, universally, that a square inch 
of malleable iron is capable of sustaining a force equal to £7 
tons, we should, in very many instances, overrate its powers. 
From the valuable and highly interesting reports which 
have been made on this branch of the subject, by the Commit- 
tee of the House of Commons, appointed for enquiring into the 
practicability of erecting a bridge over the Straits of Menai, the 
most important additions have been made to our knowledge 
regarding the strength of materials in this department of bridge- 
building. The very ingenious and effective methods also which 
have been adopted for proving the strength of iron, have been 
of the greatest consequence in forming the opinion of the en- 
gineer, and in verifying his experience. With machines of this 
kind, which we have seen in operation at the works of Captain 
Brown and Messrs Brunton in London, a force of upwards of 
100 tons, is exerted with the greatest ease and facility, and 
with an exactness and precision which is altogether astonish- 
ing in experiments of such magnitude. By facilities of this 
kind, the chain-cable manufacturer is enabled, practically, to 
ascertain what each link, shackle and bolt will sustain, and he 
now proceeds with a degree of confidence and certainty in 
these matters, which, but a few years since, rested almost wholly 
upon hypothesis. 
In one of these interesting trials at Messrs Bruntons’ work on 
the Commercial Road, we witnessed some experiments with cir- 
cular bolts of* iron, to which a strain was progressively applied 
till the machinery indicated a maximum of 92 tons. In this 
experiment, when the strain had amounted to 60 tons, it was 
observable that small particles of the oxide of iron began to se- 
parate on the surface, and when the hydraulic machine was 
wrought up to a pressure of 75 tons, the part which ultimately 
separated and gave way, became sensibly smaller, its tempera- 
ture was also somewhat encreased ; and when the register of the 
machine indicated 92 tons, it suddenly parted or broke asunder, 
VOL. V. NO. 10. OCTOBER 1821. 
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