RAILWAY CONSTRUCTION. 207 



fracture can be detected at once, and the broken chair quickly 

 replaced. 



The chair performs the very important duty of distributing 

 the weight of the rolling load on the upper surface of the sleeper. 

 If the under side or base of the chair is small, and the rolling 



' O 



load large, the chair will very rapidly wear or imbed itself into 

 the wood of the sleeper, shortening the life of the latter in a very 

 palpable manner. The short narrow chair naturally gives less 

 stability than the larger and broader chair. The chair shown in 

 Fig. 292, which was much used for 75 Ib. rails some twenty 

 years ago, has much less base area and stability than the chair 

 shown in Fig. 293, adopted for rails of a similar weight in the 

 present day. The former had a bearing surface on the sleeper 

 of only 53 square inches, as compared with 89 square inches in 

 the latter. The base area of the chair must be in proportion to 

 the weight it has to carry and distribute, and it would be false 

 economy to stint the surface area of one of the details which 

 influences so materially the stability and durability of the 

 permanent way. 



As will be seen in Figs. 294, 295, and 296, the chairs at present 

 used for 80, 85, and 90 Ib. rails have a much larger bearing 

 surface than the chair shown in Fig. 292. 



With the wider chair, a .much longer and better seat can be 

 given to the under table of rail, and a greater length of jaw for 

 holding the wooden key. The longer the rail-seat the steadier 

 the rail and the smoother the running. 



The keys are generally made of hard wood, sometimes com- 

 pressed by a special process, cut slightly taper, or wedge, shape, 

 and driven in between the jaw of the chair and the vertical web 

 of the rail. On some railways the key is placed outside the 

 rail, as in Fig. 297, and on others inside the rail, as in Fig. 298. 

 The latter method possesses many advantages over the former. 

 The outer jaw of the chair can be brought well up to the under 

 side of head of rail, giving the rail more lateral support and 

 better means of preserving the correct cant ; and, as in this chair 

 the outer jaw permanently fixes the gauge, the working out of one 

 or more of the keys does not leave the rail exposed to be forced 

 outwards and widen the gauge, as in the case with dropped keys in 

 outside keying. Another and very important advantage of inside 

 keying is that platelayers, when inspecting the road by walking 

 between the rails, can readily examine the keys on both sides. 



