440 



REPORT — 1878. 



A limited number of experiments were made with wrought-iron blocks upon 

 steel tyres, a mean of which gave the following result: — 



The following table shows the result obtained by the sliding of the wheel on the 

 rail — that is, a steel tyre on a steel rail : — 



The general results of these tables show that the co-efficient of friction between 

 moving surfaces varies inversely in a ratio dependent upon the Telocity at which 

 the surfaces are moving past each other ; probably the equation would be of the 



form of 



b + v 



The co-efficient of friction, moreover, at these velocities becomes smaller also 

 after the bodies have been in contact for a short time. That is to say, the longer 

 the time the surfaces are in contact, the smaller, apparently, does the co-efficient of 

 friction become. This result appears more marked in the case of cast-iron blocks, 

 than of the wheel sliding on the rail. This effect, however, does not appear to be 

 unnatural ; as the friction develops heat, and the consequent expansion tends to 

 close up the pores, and to make the heated surface a more united surface than the 

 colder surface. Besides which, it is probable that in the act of rubbing, small par- 

 ticles may be detached which may act as rollers between the surfaces. 



It will also be observed that the co-efficient of friction between the cast-iron 

 block and the steel tyre is much krger than that between the steel tyre of the 

 wheel and the rails, which were also generally of steel. 



As has been above mentioned, the sliding of the wheel on the rail takes place 

 when the friction of the brake blocks is greater than the adhesion between the 

 wheel and the rail, which is due to the weight upon the wheel. This was found 

 to amount generally to about 24 to 28 per cent, of the weight. 



_ The influence which these results have upon brakes for railway trains may be 

 briefly summed up as follows. 



In order to produce a given result at different velocities, the pressure applied 

 to the brake blocks must increase in the proportion shown by the co-efficient of 

 friction. 



Thus at 50 miles an hour, the pressure required to make one pair of wheels 



