152. EIGHTH REPORT — 1838. 



The longitudinal beams under the roadway are 14 inches square ; 

 and transverse joists, 3 feet 6 inches apart, and projecting about 2 feet 

 on each side, are laid across to receive the 3-inch planking, whicli 

 is covered witli a composition to form a roadway. 



The rails for the locomotive engine and train 'are raised above the 

 planks about 8 inches, on longitudinal beams or sleepers of timber, 

 about 12 inches by 6. A strong framed railing is then fixed along 

 each side, the length of the bridge, and completes the structure. 



The spandril framing is connected and bound, both to the roadway 

 and to the ribs, by means of iron bolts, straps, and keys, in the different 

 situations shown on the model. One of the radiating struts in each 

 spandril is carried on from the rib to the longitudinal beams, and con- 

 nected thereto, and to the masonry, by bolts passing through and run 

 down the piers about 8 feet. 



In this system of timber bridge building, the straight trussing in the 

 main principle of support is dispensed with ; for the spandril framing 

 must not be looked upon as such ; it is merely a combination of wood- 

 work, to convey the weight coming upon the roadway on to the sim- 

 ple curved rib ; and all timbers in a state of tension are avoided ; for 

 when a weight comes upon the roadway, the whole structure under- 

 goes compression. 



Mr. Green has also applied this laminating principle to a more durable 

 material, viz. iron, and he described the modifications which this appli- 

 cation rendered necessary, and the advantages it offered. 



Outline of the Principles of the Oblique Arch. 

 By Peter Nicholson. 



The oblique arch is an invention of comparatively recent date ; but 

 the general use of the locomotive engine rendering it urgent to pre- 

 serve the most direct line for railways, has caused the general adoption 

 of oblique bridges on all the lines of railway now in progress, and 

 it has become a matter of importance that the theory of their con- 

 struction should be fully understood. 



The principles of the oblique arch which the author proposes for 

 the guidance of engineers, require that five of the faces of each stone 

 be prepared in such a manner that four of them shall recede from 

 the fifth ; and, when the stones are arranged in courses, the surfaces of 

 the fifth face shall form one continued cylindric surface, which is the 

 intrados, and the other four surfaces shall form the beds and ends of 

 the stones on which they join each other. In every course two of the 

 opposite surfaces of the first stone, two of the opposite surfaces of the 

 second stone, and so on, shall form two continued surfaces throughout 

 the whole length of each course ; and the edge of each of these con- 

 tinued surfaces in the intrados shall be a spiral line. If a straight line 

 be di'awn through any point in one of the spiral lines, perpendicular to 

 the axis of the cylinder, the straight line shall coincide with that con- 

 tinued surface which is a bed of tliat course, and the straight line thus 



