156 Mr Buchanan's Report on a Bridge of' Suspension, 
site size and length * This will support, with perfect safety, 8 
tons upon the inch ; but at any rate the strength of every bar 
can be proved before it goes into the bridge, and rejected if it is 
too weak ; or else, as already remarked, the strong and the weak 
can be so blended together, that the strength of the compound 
chain may be rather above than below the standard. 
It will require, in all, 72 chains, each 1 \ inch diameter, to pro- 
duce that strength which I should deem necessary : that is 86 
on each side of the bridge. These I dispose in a compact square, 
having 6 chains in breadth and 6 in depth, and packed as close 
to each other as the joints will permit, so that they will not oc- 
cupy more space than a square of 15 or 16 inches. Each chain 
is composed of a series of bolts, 15 feet long each, united toge- 
ther, end to end, by a joint which is of the same construction in 
them all. This joint is formed by upsetting each end of the long 
bolt, and hammering on it a shoulder 11 inches long, and pro- 
jecting i inch all round, placing the shoulders of each two adja- 
cent bolts together, and then forming a cast-iron socket to re- 
ceive these shoulders, and hold them fast together, as seen in 
Pi. V. Fig*. 1. These sockets are cast in two halves to the exact 
mould of the shoulder and bolt, which they embrace when put 
together, and they are then bound by hoops. Cast-iron can be 
loaded with the utmost safety, with 4 tons, if not 7 tons on the 
inch, so that if the block, exclusive of the hollow socket, has 
twice the thickness, or twice the cross section of the bolts, in 
every part it will be fully as strong; and if it have live times the 
section, which will really be the case, the bolt itself will give way 
before the block. But to obviate every objection to cast-iron, 
which can be so readilv formed into those hollow blocks, while 
the malleable iron presents great difficulties in this respect, each 
joint itself can be easily proved as well as the bolt. These joints, 
in the different rows of chains, I propose disposing at equal in- 
tervals in the length, so that when viewed sideways, they will 
have a curved appearance, as seen in the principal drawing. But 
any other arrangement may be adopted that may be found more 
convenient, as this is a matter of no very great importance. 
* This iron possesses also the important advantage of withstanding exposure 
much better than the refined sort, which, in proportion to its fineness, appears to 
corrode more rapidly by the action of air and moisture. 
(To be concluded in next Number.) 
