TRANSACTIONS OF SECTION G. 



509 



It is convenient in looking at the question of railway-brakes to consider first, 

 what is the operation of a brake ? 



A train, through the adhesion of the wheels of the locomotive acting on the 

 rails, slowly accumulates energy, and for each ton of weight in the train the 

 accumulated energy is equal to 120 foot-tons at 60 miles per hour, 53 foot-tons at 

 40 miles per hour, and 30 foot-tons at 20 miles per hour. Thus, for a train of 

 fifteen vehicles, weighing 200 tons, the energy at 60 miles per hour is equal to 

 24,000 tons falling a distance of one foot. 



After a train has attained the desired speed, the reasons for stopping it may be 

 of two kinds : (1) at prearranged places for convenience ; and (2) for the pre- 

 vention of accidents or for mitigating the consequences if accidents are unavoidable. 



To stop a train for the first reason requires but a limited amount of force, which 

 may be applied in any crude manner. 



For the prevention of accidents, however, there is required : — 



a. The instantaneous application of the greatest possible amount of retarding 

 force. 



b. The continuous action of this force until the momentum of the train is 

 destroyed. 



iSSec. 



The retarding force used in practice is that due to the friction resulting from 

 the forcible application of pieces of metals or wood (brake-blocks) to the tyres of 

 the wheels ; this friction impedes the rotation of the wheels, and tends, through the 

 adhesion of the wheels to the rails, to destroy the energy stored in the train. The 

 retarding force is therefore limited to the resistance obtainable between the wheels 

 and rails. 



It was at first customary to attach to a train, for purposes of retardation, a 

 certain number of vehicles with extra weight, to which the brakes were applied ; 

 but since the question of retardation has become better understood, brakes have 

 been applied to every vehicle, the means of applying these brakes being placed in 

 the hands of both the engine-driver and the guard. The reason for this is that the 

 maximum amount of retarding force can be obtained only by applying brake-blocks 

 to every wheel in the train, each block being pressed with sufficient force to pro- 

 duce a resistance to the rotation of the wheel just equal to the greatest possible 

 friction between the wheel and the rail. This greatest possible friction occurs 

 when the adhesion of the wheel to the rail is just about to be overcome by the 

 superior effort of the brake-blocks, which effort, if further increased, immediately 

 begins to stop the rotating movement of the wheel, and thus causes it to slide 



