o 
IRON BRIDGES. 
New method In order to render this evident, I have made a drawing to 
wi.hoiitsnp^ s ^ ew ’ n what manner the centering or frame for an arch of this 
ports beneath, magnitude may be constructed ; hitherto the centering has 
been made by placing supports and working from below ; but 
in the case of the Menai. from the nature of the bottom of the 
channel, the depth at low water, and the great rise and rapidity 
of the tides, this would be very difficult, if not impracticable. 
I therefore propose changing the mode, and working entirely 
from above; that is to. say, instead of supporting, T mean to 
suspend the centering. By inspecting the drawing, the general 
principle of this will be readily conceived. 
S he abut- I propose, in the first place, to build the masonry of ihe 
abutments as far back as fhe lines AB, CD, and in the particu- 
lar manner shewn in the section. 
Having carried up the masonry to the level of the roadway, 
I propose, upon the top of each abutment, to construct as 
many frames as there are to be ribs in the centers, and of at 
least an equal breadth with the top of each rib. These frames 
to be about fifty feet high above the top of the masonry, and 
to be rendered perfectly firm\ and secure. That this can be 
done, is so evident, I avoid entering into details respecting the 
mode. These frames are for the purpose of receiving strong 
blocks, or rollers and chains, and to be acted upon by windlasses 
or other powers. 
Centering of I next proceed to construct the centering itself ; it is pro- 
fuur separate” P 0sec ^ t0 made of deal baulk, and to consist of four separate 
ribs carried ribs, each rib consisting of a continuation of timber frames, 
cm from tae fj ve f eet j n across the top and bottom, and varying in 
each side, and depth from 25 feet, near the abutment, to 7 feet 6 inches at 
supported by the middle or crown. Next to the face of the abutment, one 
eitaiiiS. 
set of frames about 50 feet in length can, by means of temporary 
scaffolding and iron chain bars, be readily constructed and 
fixed upon the masonry offsets of the abutment, and to hori- 
zontal iron ties laid into the masonry for this purpose. A set 
of these frames ffour in number) having been fixed against the 
face of each abutment, they are to be secured together by cross 
and diagonal braces ; and there being spaces of only 6 feetB 
inches left between the libs (of which these frames are the com- 
mencement) they are to be covered with planking, and the 
whole converted into a platform 50 feet by 40. By the nature' 
of 
