155 
proposed to be erected at Montrose. 
bridge, the chains are formed by round bars or bolts, 15 feet 
long each, and 2 inches in diameter, each weighing about 2 cwt. 
The advantage of these large and heavy bars is, that they save 
joinings ; but they are attended with other inconveniences, which 
more than counterbalance it. I should prefer bolts of smaller 
diameter, not exceeding 1 \ inch ; these can be more easily pro- 
cured, are easier wrought, and easier put together, so that on the 
whole, they are attended with less expence. Another advantage 
arises from the smallness of these bars ; their great number en- 
ables us to blend together in every part of the compound chain, 
tlie various qualities of the iron, so that if some bars should be 
below their proper strength others will be as much above it, and 
the collective strength of the whole will still be in every part of 
the arch rather above than below the standard. A greater num- 
ber of joints will no doubt be required for the small bars ; but 
this is a matter of little importance, more especially as these can 
be made of so simple a construction that the bars can be put to- 
gether and taken to pieces again with the greatest facility. * 
Another important consideration regards the quality of the 
iron to be employed. Iron, it is well known, varies greatly in 
this respect, and may be had of very different degrees of strength 
and toughness, according to the pains bestowed in rolling and 
hammering it. But the additional strength acquired by this ela- 
borate process, is less in proportion than the increased expence 
incurred by it ; and this refined iron, therefore, is only necessary 
in cases where the whole stress is laid, and the whole strength 
depends, on a single or double bolt, and where also it is desirable 
to combine as much strength as possible with little weight. But 
in the proposed bridge, the strain will be divided in every part 
of the arch from end to end among 70 bolts and upwards, and a 
weight of 30 or 40 tons additional will scarcely be felt, and can 
produce no sort of inconvenience, but is rather of advantage, as 
it increases the mass of our structure, and adds to its steadiness. 
I should prefer, therefore, common rolled iron of good quality, 
and such as can be had ready formed in bars or bolts of requi- 
* The roundness of the bar possesses also an advantage in exposing less surface 
in proportion to its solidity to the action of the atmosphere, than any other form. 
This at least renders it, under the same circumstances, preferable to square or rect- 
angular bars. 
