IRON BRIDGES. 
striking the centering, will be rendered gradual, and perfectly 
safe : insomuch, that this new mode of suspending the 
centering instead of supporting it from below, may, perhaps, 
hereafter be adopted as an improvement in constructing iron 
bridges, even in places not circumstanced as are the Menai 
Straits. Although the span of the arch is unusually great, yet, 
by using iron as a material, the weight upon the centre, when 
compared with large stone arches, is very small ; taking the 
mere arch stones of the centre arch of Blackfriars bridge at 
156 x 43 X 5 equal to 33,540 cubic feet of stone, it amounts to 
2,23 6 tons, whereas the whole of the iron-work in the main 
ribs, cross-plates and ties, and grated covering plates, that is to 
say, all that is lying on the centering at th« time it is to be 
eased, weighs only 1,791 tons j it is true, that from the flatness 
of the iron arch, if left unguarded, a great proportion of this 
weight would rest upon the centering j but this is counter- 
balanced by the operation of the iron ties in the abutments, 
and wholly commanded by the suspending chain-bars. 
When the main iron ribs have been completed, the next step Intermediate 
is to proceed with the iron supporters of the roadway: and supporters of 
. rr . tli e roadway 
these, instead of being constructed in the form of circles, or framed trian- 
that of perpendicular pillars, as hitherto, are here a series of gular. 
triangles, thus including the true line of bearing. These 
triangles are, of course preserved in a vertical plane by cross 
ties and braces : iron bearers are supported by these triangles, 
and upon the bearers are laid the covering plates under the road- 
way, which instead of being solid are (in order to lessen the 
weight) proposed to be reticulated. 
If I have throughout this very succinct description, made 
myself understood, it will I think be admitted, that the con- 
structing a single arch across the Menai, is not only a very 
practicable but a very simple operation and that it is rendered 
so, chiefly by adopting the mode of working from each abut- 
ment, without at all interfering with the tideway. 
METE- 
