1824.] 
inhabitants of populous districts to 
exert their influence, and, by calling 
public meetings in their respective 
neighbourhoods, to canvass this new 
project, and promote its extension from 
town to town. \ The profit to be derived 
by the public from the scheme, may be 
computed from the enormous annual 
expenditure now wasted in purchasing 
and feeding unnecessary horses. 
I remain, Sir, 
Most respectfully yours, 
Tuomas Gray. 
Nottingham, 1st Sept. 1824. 
LL Ree 
For the Monthly Magazine. 
> SUSPENSION BRIDGES. 
R. BUCHANNAN, the able engi- 
neer, has been commissioned by 
the public-spirited inhabitants of Mon- 
trose, in Scotland, to draw-up a re- 
port for a projected bridge, to be made 
of wrought iron, on the suspension 
plan, across the mouth of the South-Esk 
river, on the great road from Edinburgh 
to Aberdeen. The designof Mr. Bu- 
channan varies, in some respects, both 
from that of Mr. Telford in the sus- 
pension bridge across the Menai strait 
at Bangor, and from that of Captain 
Brown across the Tweed at Berwick. 
The printed report of Mr. B. contains 
a more luminous view of the principles 
of bridge-building, upon this new plan 
of suspension, than I have previously 
seen. His estimate of the strength of 
wrought iron not only proves him to be 
well qualified for the task, but it is at 
the same time satisfactory to the pub- 
lic to be well assured, in all such cases, 
that the practical results do not de- 
pend on any uncertain contingencies, 
It is not enough to shew that sucha 
stupendous erection as an iron bridge 
700 or 800 feet in length may not fall, 
it must be shown, and even proved to 
demonstration, that it cannot fall: and 
so far T think Mr. B. has, in his report, 
succeeded. It is far too long and minute 
for your columns, and I must therefore 
give your readers an outline only of 
the proposed plan. 
It is well known that a great portion 
of the strain which is exerted on the 
main chains of a suspension bridge is 
owing to the vertical pressure, or the 
weight of the material which forms the 
chain. This is indeed so obvious, that 
an ordinary rope, which is not more 
# profit, or a canal without water.—Gore’s 
—* Advertiser, Liverpool, 12th August, 
$24. 
Suspension Bridges. 
227 
than 1°8 the specific gravity of iron, 
will not bear tension many feet in 
length in horizontal direction, without 
forming a curve. In the design of every 
bridge on this construction, therefore, 
considerable allowance must be made 
for the strain on the chains, both from 
the weight of the iron employed, as 
well as the strain on the abutments, or 
what might be called the lateral strain ; 
to which must be subsequently ‘added, 
the probable weight the bridge is cal- 
culated to sustain in the construction 
of the necessary road-way} and the 
passengers or loading which it may 
have to support at any one period. 
After computing the weight, collec- 
tively, from these sources, at the in- 
tended bridge of Montrose, to be some- 
what less than 1,000 tons, Mr. B. pro- 
poses to have seventy-two chains, thirty- 
six on each side of the road-way, formed 
intoacompact or solid square of six by six; 
these chains to be made of iron rods, 
about one inch and a-half diameter and 
fifteen feet in length, connected together 
by strong joints, of a peculiar construc- 
tion. These chains he calculates as being 
capable of sustaining a pressure of 
2,000 tons, or double that of the ag- 
gregate pressure of the weight of the 
materials of the bridge and its loading. 
The chief variation, however, in the 
design of Mr. Buchannan from that of 
the Tweed bridge, is in the greater 
height of the pillars, or towers, over 
which the chains are suspended. This 
elevation of the towers allows the chains 
to have a greater or deeper inverted 
curve, by which the strain upon the 
tenacity of the metal is materially di- 
minished. The bolts or links of his 
bridge will also be of lighter materials 
than those of the bridge at Berwick, 
owing to the diminished pressure (in 
proportion to its length) from the ad- 
ditional curve. There canbe no ques- 
tion, I apprehend, as to the greater eco- 
nomy of materials and of aggregate 
strength, though the labour will be 
greater from the combination of a large 
number of smaller chains, than from a 
small number of large chains. If a 
large chain were to break, the conse- 
quences would probably be: destructive 
to the entire fabric; but in a great num- 
ber of chains combined the strain on 
the materials will be so much divided, 
that the stretching of any individual 
chain, or an imperfect link, will be op- 
posed by all the other chains with 
which it is in connexion, Iam of opt 
nion that suspension bridges will ul- 
2G 2 timately 
