TRANSACTIONS OF SECTION Q. 



513 



brake-block pressure, showing that, through some cause not yet fully determined, 

 the holding power of brake-Hocks 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 apparent in diagram 5. Hence the question of the proper 

 amount ol brake-force needed at each instant, during the time required to stop a 



/v? 6 



Seconds o t '-' 3 ■* 5 6 7 a 31011 1213 HUH 17 181320 215223 va2fi -a u 3330 



train, is still further complicated by this decrease which occurs in the coefficient 

 of friction after the brakes have been applied, and which results fro m the time 

 during which they are kept applied, irrespective of any change in speed, This de- 

 crease in the coefficient of friction is shown in the following table : — 



Coefficient of Friction as affected by Time. 



Diagram 7 shows the curves of this decrease obtained from a few of the ex- 

 periments. It would seem as if the coefficient of friction due to each speed be- 

 comes nearly uniform after a certain number of seconds have elapsed. The ex- 

 periments were, however, necessarily limited 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 over- 

 comes 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 prac- 

 tically uniform speed, as obtained by means of an increasing brake-block pressure. 

 The line p shows the pressure, f the friction, and s the speed, which decreased 

 slightly during the experiment, and would have caused an increase in the coeffi- 

 cient of friction had it not been counteracted by the element of time. 



1879. 



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