.1818.] across the Mersey, at Runcorn. 15 



rupted, even during the erection of the bridge. For this reason 

 $t is supposed to be laid down as data by the committee that the 

 middle opening of this bridge shall not be less than a thousand 

 feet, and that the road or arch shall be so far elevated as to leave 

 of that width a clear space of not less than 70 feet in height 

 above high water mark. 



The bottom and sides of the river are a compact sand-stone 

 rock ; the section at Runcorn is agreeable to that given in an 

 engraving of a design for a bridge in this situation by Mr. Tel- 

 ford, and of that copy which was presented by him to the Royal 

 Institution in April last. The design now submitted consists of 

 a road constructed of cast and wrought iron, suspended in 

 equilibrio from cast-iron fulcrums or piers, but differing from 

 other roads suspended from catenarian arches (from Mr. Telford's 

 and Capt. Brown's, for example) in the form of, and distribution 

 of the weight on the piers, in the suspension of each part singly 

 and directly from the piers, and as to a mode of deriving fixed 

 points of resistance from the rock, in lieu of abutments of 

 masonry. 



Description. 



Of the 18 figures referred to, the first three are descriptive of 

 the general appearance of the bridge ; the fourth to the 10th, of 

 the details of its construction ; and the remainder are chiefly 

 diagrams explanatory of the principles on which it acts. In 

 Fig. 1, elevation : Fig. 2 X j)lan : and in Fig. 3, profile, 



A, represents the fulcrums or piers ; being connected ranges 

 of hollow tapering columns, or cylinders of cast-iron, placed 

 perpendicularly, and braced by stays, struts, tie plates, &c. 

 (See Fig. 3.) " 



B, the jointed iron rods which suspend the road, and which 

 are not fixed to, but merely lie loose on, the ridge piece of the 

 fulcrums in grooves, as seen in Fig. 3. That part which slides 

 or passes over the fulcrums is composed of short links, so as to 

 6lide with little friction. 



C, iron rods continued from where the others are attached to 

 the road, down to the stratum of rock, and there fixed. These 

 rods are intended as a substitute for an abutment of masoniy, 

 which would otherwise require to be formed at the end of the 

 road, of such power as to resist the strain of the road indicated 

 by the diagonals 1 to 13 from the fulcrum to the abutment, and 

 amounting, even when the bridge is unloaded, to 450 tons. 



Were there no rock, then a stratum of connected blocks of 

 stone, wood, or cast-iron, might be substituted, at such a depth 

 as tl at the load of earth over them might produce the power 

 required, as counterpoise. 



Or, supposing the ground to be soft, and of uniform texture, 

 for a considerable depth, then might piles be driven and con- 

 joined a few feet below the surface of the ground by a pavement 



