IRON BRIDGES. 
3 
of the framing, and from its being secured by horizontal and 
suspending bars, I presume every person accustomed to practical 
operations will admit that these platforms may be rendered per- 
fectly firm and secure. 
The second portion of the centering frames, having been facing of the 
previously prepared and fitted together ip the carpenter’s yard, tfons'of* the 
are brought in separate pieces, through passages, purposely left fi'ames 
open in the masonry, to the before-mentioned platform ; they 
are here put together, and each frame raised by the suspending 
chain-bars and other means, so that the end, which is to be 
joined to the frame already fixed, shall rest upon a small 
moveable carriage : it is then to be pushed forward, perhaps 
upon an iron rail-road, until the strong iron forks which are fixed 
upon its edge, shall fall upon a round iron bar which forms the 
outer edge of the first or abutment frames : when this has been 
done, strong iron bolts are put through eyes in the forks, and 
the aforesaid second portion of frame -work is suffered to 
descend to its intended position by means of the suspending 
chain-bars, until it doses with the end of the previously fixed 
frame, like a ruie joint. Admitting the first frames were firmly 
fixed, and that the hinge part of this joint is sufficiently strong, 
and the joint itself about 20 feet deep, I conceive that even 
without the aid of the suspending bars, that this second portion 
of the centering would be supported ; but we will for a moment 
suppose, that it is to be wholly suspended. — It is known by 
experiments, that a bar of good malleable iron, one inch 
square, wilt suspend 80,000lbs. and that the powers of suspen- 
sion are as the sections ; consequently a bar of If inch square 
will suspend I80 ; 000lbs. but the whole weight of this portion 
of rib, including the weight of the suspending bar, is only 
about 30,000lbs or one-sixth of the weight that might be 
safely suspended ; and as I propose two suspending chain-bars 
to each portion of rib, if they had the whole to support they 
would only be exerting about l-12th of their power ; and con- 
sidering the proportion of the weight which rests upon the 
abutments, they are equal also to support all the iron-woik ot 
the bridge, and be still far within their power. 
Having thus provided for the second portions of the center' end the others, 
-ing a degree of security far beyond what can be required, 
similar operations are carried on from each abutment, until the 
B 2 parts 
