pr oposed to be erected at Montrose. 273 
ed in the firmest manner. The hollow within is built up with 
stone, and while the weight of the bridge will chiefly rest on 
the masonry, the iron-casing will both keep the stone together, 
and will be ready to resist any lateral force which might act on 
the pillars. The top of the pyramid is surmounted with a 
strong iron-plate, over which the chains are bent, and on which 
they rest. The flexibility of the pillar will render the use of 
rollers unnecessary. The bottom of the pedestal rests on, and 
is firmly bolted to a strong bottom of iron cast in one piece, 
and from this bottom there descend a number of strong iron 
bolts, such as compose the main-chains ; these are either made 
fast to the platform of piles, or, what would be still better, to a 
strong plate or hoop of iron, embracing the whole compass of 
the base of the pillar. In this manner a connected chain of 
iron will be formed, from the base to the top of the compound 
pillar, and these iron-bolts will also serve to bind the mason- 
work together. If the pedestal be 11 feet square, it may be 
formed wholly of stone. The pyramid will then be bolted to a 
strong cast-iron plate laid on the top of the pedestal, and united 
to a similar plate on the bottom of the pedestal, by bolts pass- 
ing through the mason work ; this second plate being connected 
with the third one at the bottom of the pier, by similar bolts 
passing through the pier from top to bottom ; and this last plate 
being firmly attached to the platform on the top of the piles. 
To save metal, these plates are cast with four open squares in 
the middle. 
The two pillars, one on each side, support the whole weight 
of the bridge, and this, in the most extreme case, amounts to 
about 1000 tons, or about 500 tons to each pillar. But, as the 
base of the pillar is spread over a surface of upwards of 500 
square feet, the pressure on the top of the piles will not amount 
to one ton on the foot, which is only equal to that of a wall 13 
feet high. In fact, the foundation of the present bridge is sup- 
porting, and has supported since its erection, a load of three 
tons on each square foot ; so that this puts an end to every idea 
of the pillar sinking under the load of the bridge. Neither can 
it be crushed by this load ; for, as there are 15 or 16 feet square 
of stone at the top of the pyramid, where it is weakest, this of it- 
self will be amply sufficient to withstand, without injury, 500 tons, 
vol. xi. no. 22. oct. 1824. 
s 
