254 ! Mr Stevenson's Description of Bridges of Suspension, 
This trial was made with a rod of Welch iron which Mr 
Brunton, on examining, and judging both from the strain ap- 
plied, and the appearance of the fracture, concluded was not of 
the first quality. 
It is hardly necessary, in chain-bridges of great extent, to ob- 
serve, that no danger whatever is to be apprehended from the 
ordinary weight of the passing load. But, in projects of this 
kind, it becomes necessary to consult the safety of the fabric in 
extreme cases* by taking into calculation the crowded state of 
the roadway, when a multitude of people or a drove of cattle 
passes it. The former, however, we consider to be capable of 
inducing a greater weight, and of being much more danger- 
ous, and perhaps less under controul, under certain circumstan- 
ces, than the latter. We find that a given area, closely covered 
with men, will have a greater weight than the same space occu- 
pied by cattle, in the proportion of about 9 to 7 ; and every one 
knows that a drove of cattle is more likely to be gradually ad- 
mitted upon the roadway of such a bridge, than a mob of people, 
whom objects o£ interest attract to any particular spot. Of 
this, a remarkable instance occurred at the opening of Cap- 
tain Browns bridge over the Tweed in July 1820, when it was 
intended to keep the roadway clear for the ceremony of the day; 
but this proved quite impracticable, and a crowd of people broke 
through every obstruction, and forced their way upon the bridge ; 
and it was estimated, that at one time there were about 700 
people upon the roadway. Now, taking each person and this 
number at 150 lb., it would give about 47 tons, besides its own 
weight which it sustained, without any apparent derangement. 
The main chains of the Union Bridge, as we have seen, 
are formed of circular rods of iron, measuring about two 
inches in diameter ; and a bar of that strength, which was 
not considered of the first quality, sustained a force equal 
to 92 tons. As the number of catenarian chains in the Union 
Bridge are twelve, if we estimate the strength of each rod, with 
its shackle, at 92 tons, we shall have the aggregate strength 
of the whole to be 92 x 12 1104 tons. We have already sup- 
posed the weight of the bridge, between the points of suspension, 
to be equal to 100 tons, and if to this we add 47 tons, as the 
greatest load which is likely to be ever brought upon it at once, 
