TRANSACTIONS OF SECTION G. 



511 



gradient of 1 in 264— the strain on the draw-bar being also measured during the 

 experiment. In this case the strain on the draw-bar diminished in the same ratio 

 as the friction. 



From this it is evident that the retardation which arises when the wheel is 

 sliding on the rail is far less than the retardation produced by the effect of the 

 brake-blocks when applied to the wheels so as to allow the wheels to continue 

 revolving;. 



In order to understand this it is necessary to consider the general action of 

 railway brakes. When a train is moving at a given velocity the adhesion of the 

 wheels on the rails causes them to revolve ; every point on the surface of the tyre 

 moves round at the same rate as that at which the train itself is moving forward ; 

 but every such point in relation to the forward movement of the train comes suc- 

 cessively to rest at the moment when it comes in contact with the rail. Now when 

 the brake is applied with a slight pressure only, the wheel continues to move round 

 at the same rate as that at which the train is moving, but it moves with more 

 difficulty, and this increased difficulty in moving is shown either by an increase 

 in the tractive force required to keep up the forward motion, or, in cases where the 

 accelerating force is not kept up, by the tendency of the moving mass to come to 

 rest in a shorter time than would otherwise be the case. But if the pressure 



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with which the brake is applied be increased, a point is reached when the friction 

 between the brake-block and the wheel first approaches, then equals, and finally 

 exceeds, the adhesion of the wheel on the rail. When this happens, the wheel 

 first begins to revolve more slowly, and then ceases to revolve and slides along the 

 rail, or, as it is usually termed, is skidded. The retardation is then no longer due to 

 the friction between the brake-block and the tyre of the wheel ; but the vehicle is 

 transformed for the time from a vehicle on wheels into a sledge, and the retarda- 

 tion is due to the excess of resistance which is produced by making the vehicle 

 slide along the rails over that produced by making the vehicle move forward on 

 wheels revolving freely. 



The reason why the retardation caused by the brake-blocks applied to revolving 

 wheels exceeds that caused by the skidded wheels became obvious from the fact 

 next discovered, viz., that the coefficient of friction between the brake-blocks and 

 the wheels varied inversely according to the speed of the train, a higher propor- 

 tionate percentage of brake-block pressure being required to obtain a given amount 

 of friction at high speeds, and a lower pressure at lower speeds. 



This is illustrated by the diagrams 4, 5. In these diagrams p represents pres- 

 sure, f friction, and s speed, measured on the respective scales at the side, to be 

 corrected by the multiple before mentioned. It will be observed that the ratio of 



