164 LARY BRIDGE. 



the arches, arising from change of temperature, may be 

 provided against. 



These bars are cast in the form of the letter T, and 

 fixed vertically over the main ribs, and are conse- 

 quently supported between the pier frames, and their 

 intersections with the ribs, by the triangular braces 

 before described ; forming a flat arch for the whole 

 length between the abutments of the bridge ; the radius 

 of which, as before stated, is 4670 feet. On these bars 

 rest the covering or roadway-plates, 1 inch thick, 13 

 feet long, and 3 feet wide. They are laid in the trans- 

 verse direction of the road-way, and are connected 

 , laterally by flanges and screw-bolts, and transversely 

 by dovetails ; forming one sheet of cast iron, 26 feet 

 wide and 500 feet long ; which being firmly connected 

 to the framing below, by strong dove-tail tenons, cast 

 to the bearers, and wedged tight in corresponding mor- 

 tices in the plates, serves the double purpose of road- 

 way-plates and bracing, to the whole of the framing, 

 by which it is supported. A bed of strong clay, four 

 inches thick, covers the whole surface of these plates, 

 upon which is laid small broken stone in the usual way 

 of making roads. Iron side-railings, and foot-ways of 

 Cann slate, complete the superstructure. 



In the construction of bridges of every description, 

 it is of the utmost importance, that the directions of the 

 joints of the several parts composing the arch, should 

 be at light angles to the line formed by the several 

 forces acting upon the arch, by means of its own and 

 superincumbent weight : it is also important to the sta- 

 bility of the arch, that the line of thrust should pass 

 through the whole of the parts composing the arch, in 

 as nearly as possible, the same relative point : the two 

 postulates of an arch of equilibrium, about which so 

 much has been written, with what success to the practi- 

 cal engineer, I must leave others to determine. The 

 labours of Dr. Hutton, should however be appreciated ; 

 for, in a mathematical point of view, his theorem for an 

 arch of equilibrium is clear and satisfactory, though 

 the practical difficulty (or I would say impossibility) 



