4 
IRON BRIDGES. 
parts are joined in the m.Jdle and form a complete centering ; 
and being then braced together, and covered with planking 
where necessary, they become one general platform or wooden 
bridge on which to lay the iron-work. 
Upon this It is, I presume, needless to observe, that upon such a center- 
arch 4 tse^ 01 P^ al ^ orm the iron-w r ork, which is understood to have been 
laid. previously fitted, can be put together with the utmost correct- 
ness and facility ; the communications from the shores to the 
centering will be through the before-mentioned passages left in 
the masonry. 
Main ribs and The form cf the iron-work of the main ribs will be seen by 
framed con- the drawings to compose a system of triangles, preserving the 
nection at top. . . , 5 . _ , 1 . . ° v ,. & T . 
principal points of bearing in the direction or the radius. It is 
proposed in the breadth of the bridge (i. e. 40 feet) to have 9 
ribs, each cast in 23 pieces, and these connected by a cross- 
grated plate, nearly in the same manner as in the great 
Aqueduct of Pontcysylte over the valley of the Dee, near 
Llangollen ; the fixation of the several ribs in a vertical plane 
appearing (after the abutments) to be the most important object 
in iron bridges, I propose to accomplish this by covering the 
several parts, as. they are progressively fixed, with grated or 
reticulated and flanched plates across the top of the ribs. This 
would keep the tops of the ribs immovable, and convert the 
whole breadth of the bridge into one frame j besides thus 
securing the top, I propose also having cross braces near the 
bottom of the ribs. 
Consideration The main ribs being thus fixed, covered and connected 
of the pressure, together, the great feature of the bridge is completed ; and as, 
from accurate experiments made and communicated to me by 
my friend the late William Reynolds, of Coalbrook Dale, it 
requires 448 ; 000lbs. to crush a cube of J inch of cast iron of 
the quality called gun metal, it is clear, that while the ribs 
are kept in their true position, the strength provided is naore 
than ample. 
Method of eas- When advanced thus far, I propose (though not to remove) 
ing -^and^ne "^ et to ease tim ^ er cent ering, by having the feet of the 
rat advantages centering ribs (which are supported by off-sets in the masonry 
of this general 0 f the front of the abutment) placed upon proper wedges; 
the rest of the centering to be eased at the same time by means 
of the chain bars. Thus, the hitherto dangerous operations of 
striking 
