July 24, 1879] 



NATURE 



293 



to slide upon the rail. The experiments ivcre made with 

 the object of measuring the force thus brought into 

 action. 



The first result of the experiments was to show con- 

 clusivelv that the retarding effect of a wheel sliding upon 



perlieiw 



N° I. 



<~"\ Experiment IT? 27. 



\ Siip Stop yvvAout sk-vdtiiJu 



\Sap Mop witiwxa sh-uuiou) 

 \ Gradient fallmx) 1 in ]OSs. 



iSSec. 



a rail is much less than when braked with such a force as 

 would just allow it to continue to revolve. 



The annexed copies of two sets of diagrams (No. I 

 and No. 2) taken during the experiments show, more 



N" 2. 

 Eocperijnmt JIf 18. 

 ~p-^SlipSlop K-ith S?;uhiuig 

 "^ CrriulieiU^ levvl 



lb.: 



llhn 



'"1 



clearly than can be explained, the difference in the re- 

 tarding force before the wheels begin to slide upon the 

 rails, and after. These two experiments were made with 

 a single van slipped from the engine, the brakes going on 



E<icperimmtlf? «.~''-.^_ 

 • \ Gradurtxt risiriQ 1 in 264 



so 

 ec- 



.1* 



A" 



Speed of Van 



20 25 3D 5fC. 





automatically when separation from the engine took 

 place. S is a line showing the speed of the van at each 

 instant, the scale for which is at the left side. I' is the 

 pressure against four blocks acting upon one pair of 



wheels ; the vertical height of P by the scale on the right 

 hand multiplied by 240 gives the total pressure in pounds 

 on the four blocks. F is the line showing the retarding 

 effect of the four blocks upon the one pair of wheels 

 before the wheels began to slide upon the rails ; and / 

 shows the effect while the wheels were sliding upon the 

 rails. The vertical height of F or /, according to scale B, 

 multiplied by 60, gives the retardation in pounds. It will 

 be seen that the stop was made in half the time with the 

 wheels braked but not skidded of that required when the 

 wheels were skidded. 



The accompanying Diagram 3 shows in another way 

 the comparative retarding effect of the brakes when 

 acting on the revolving wheels and when applied with 

 sufficient force to skid the wheels. 



This experiment was made by keeping the van at a 

 uniform speed on a rising gradient of 1 in 264 — the line T 

 shows the strain on the draw-bar during the experiment. 

 The line .S shows the speed of revolution of the braked 

 wheels, when the revolution was checked and the friction 

 diminished as shown by the line /; the strain, T, on the 

 draw-bar diminished in a corresponding ratio. 



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 



U/^ 



AT 



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 successively 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 in- 

 crease in the tractive force required to keep up the for- 

 ward 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 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 



