proposed to be erected at M-ontrose, 
the arch is sprung, the whole rests or stands on these supports* 
and on this account the equilibrium of the arch is of an unsta- 
ble and precarious nature, so that whenever it is disturbed be- 
yond a certain point, it is sure to be completely overturned. 
When the bridge, therefore, is overloaded in any part, the fall 
of the whole structure is inevitable. The suspended arch is the 
reverse of this, and has, in this respect, the advantage. There 
is here also an equilibrium between the different parts of the 
structure, but as the materials of the arch and its roadway lie, 
in this case, below the suspending pillars, the whole rather 
hangs than rests on these pillars, and the equilibrium is, on this 
account, remarkable for its stability. It is that kind well known 
in mechanics under the name of the Stable Equilibrium ; while 
the other, being less sure, is entitled the Tottering Equilibrium. 
The one cannot be overset, but the other requires considerable 
skill to be maintained. While the raised arch, therefore, falls 
irretrievably in consequence of any overload, the hanging arch 
cannot give way until the materials themselves which compose 
it are torn in pieces. The one tumbles long before the arch 
stones are strained to their utmost ; while the other carries the 
load on its chains to the very moment of their fracture. In the 
one case the materials may be driven out of their place, and thus 
the structure may be overturned ; while in the other, we have 
the whole strength of the materials as a security against acci- 
dents. And although the common bridge is, no doubt, seldom 
known to fail, its strength and solidity are chiefly owing to the 
enormous mass of materials which compose it; while the sus- 
pended bridge, on the other hand, is distinguished by the light- 
ness of its structure, and the apparent boldness of its design. 
But malleable iron possesses a peculiar advantage in its tena- 
city; it stretches long before it actually breaks; it begins to 
stretch, indeed, with about one-third, or, at most, one-half of 
the weight which would tear it asunder ; and, as it is unsafe to 
load it with any more than this third or half of its utmost 
strength, at least if we wish to give permanence to the structure, 
this circumstance ensures the highest degree of security that can 
be desired : for even if the strength of the iron should happen 
to be estimated too low, which, however, cannot take place, if 
we are careful to prove each link of the chains before they are 
K 
