294 



NATURE 



\jfuly 24, 1879 



a vehicle on wheels into a sledge, and the retardation is 

 due to the excess of resistance which is produced by- 

 making the vehicle slide along the rails over that pro- 

 duced 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 accord- 

 ing to the speed of the train, a higher proportionate 

 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- 



lij-iji 



«°5. 



' I ' I ' I ' I ' r 



StamdSO 1 2 3 4 3 6 18 9 10 -ttrtlS U 1Hef7.a<S20 2f222lll iSXnaiSllI 



If 



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 r to p in 

 Diagram 4 with a speed of eleven miles per hour is much 

 larger than that of F to P in Diagram 5 with a speed of 

 fifty-five miles per hour. 



The following table shows the coefficient of friction 

 obtained from these experiments at rarying speeds 

 between cast-iron brake blocks and steel tyres : — 



same at the same speed, some simple rule might be 

 deduced which would gire the pressure required at each 

 speed for obtaining a certain amount of retardation ; but 

 when the speed of the van was kept nearly uniform by the 

 effort of the engine, the friction of the blocks decreased ; 

 and this occurred notwithstanding a continued increase 

 of the brake-block pressure : showing that, through some 

 cause not yet fully determined, the holding-power of 

 brake-blocks at all speeds is considerably less after some' 

 seconds of application than when first applied. This 

 peculiarity is illustrated by Diagram 6, and is also- 



If the position of the brake-blocks were always the 



apparent in Diagram 5. Hence the question of the 

 proper amount of brake-force needed at each instant, 

 during the time required to stop a train, is still further 

 complicated by this decrease which occurs in the co- 

 efficient of friction after the brakes have been applied, 

 and which results from the time during which they are 

 kept applied, irrespective of any change in speed. This 

 decrease in the coefficient of friction is shown in the 

 following table : — 



Coffficient of Friction as affected by Time 



Speed. 



Miles per 



hour. 



27 



37 



47 

 60 



After 20 

 secjnds. 



•099 

 •072 



Diagrarn 7 shows the curves of this decrease ob- 

 tained from a few of the experiments. It would seem 

 as if the coefficient of friction due to each speed becomes 

 nearly uniform after a certain number of seconds have 

 elapsed. The experiments were, however, necessarily 

 hmited to something between twenty and thirty seconds 

 each, so that this point has not been fully determined. 



The decrease in the coefficient of friction, arising from 

 time sometimes overcomes the increase in the coefficient 

 of friction arising from a decrease in speed ; especially 

 when, either from the stop being on a descending gradient 

 or from a small proportion of the train only being fitted 

 with brake power, the train takes considerable time in 

 coming to rest. Therefore, a higher brake pressure is 

 required in such cases than when the stop is made in a 

 short time. 



The accompanying diagram (8) shows a uniform 

 force of friction with a practically uniform speed, as ob- 

 tained by means of an increasing brake-block pressure. 

 The line P, shows the pressure, f the friction, and S the 



