proposed to be erected at Montrose. 149 
Next, however, to the perfect security of every part of the 
bridge, we must look to the accommodations ; and in this respect 
the suspended bridge is every way on a par with the common 
bridge, excepting that the flexibility of the arch subjects it to a 
certain unsteadiness, which, in some of these structures, has been 
felt as an inconvenience. This, however, has chiefly arisen from 
want of the proper means having been taken to prevent it ; and 
if such precautions be used, as I shall afterwards describe, I 
have no doubt this inconvenience will be completely removed. 
In the erection of a suspended bridge, the first object is to as- 
certain the utmost strain to which the chains will ever be sub- 
jected. The strain arises from the weight of the roadway, and 
of any load of carriages or people that may be laid upon it, to- 
gether with the weight of the rods by which it is suspended 
from the main chains, and also the weight of those chains them- 
selves ; each and all of these together being sustained by the co- 
hesive strength of the iron, of which those principal chains are 
composed, and which is stretched in every part of it, exactly as 
if the chain were hung perpendicularly, and a weight, correspond- 
ing to the strain on the bridge, suspended at its lower extremity. 
It is of the first importance, therefore, to know the total amount 
of all these different weights, that we may be able to apply a 
chain, whose collective strength may be amply sufficient to sus- 
tain it permanently, without stretching or altering, by the strain, 
the natural texture of the metal. In the bridge now designed, 
the roadway is to be 30 feet wide, 28 feet clear of the chains and 
side-rails, and 20 feet clear of the two footpaths, each of which, 
therefore, is four feet wide. For the support of the roadway, 
there are to be only two sets of main chains, one at each side of 
the bridge, and running in a line above the outside of the foot- 
path, and above the side-rails ; so that between the opposite rail- 
ings of the bridge all is clear foot and carriage way, the foot- 
paths being only raised above the carriage way, and protected 
from the encroachment of carriages, by short iron- posts planted 
at convenient distances, and connected together, if necessary, by 
a chain, over which one may easily step. 
I propose making the roadway wholly of iron, consisting of a 
series of iron-plates, supported on a frame-work below, and co- 
vered above with a coat of gravel or small metal, four inches 
