TRANSACTIONS OF SECTION A. 



439 



wheels to which brakes are applied continue to revolve at the rate of rotation due 

 to the forward movement of the train, the effect of the brake is to create retarda- 

 tion by the friction between the block and the wheel ; but when the pressure 

 applied to the blocks causes the friction to exceed the adhesion between the wheels 

 aud rail, the rotation of the wheels is arrested, and the wheel becomes fixed and 

 slides on the rail, being held in its fixed position by the brake blocks. 



Therefore the experiments give the co-efficient of friction — 1st, between the 

 brake blocks and the wheel, which is equal to 



the tangential force 

 the pressure applied ' 

 2nd, between the wheel and the rail, which is the 



friction of the brake blocks 

 weight upon the wheels 



It has been generally stated that there is no difference in the co-efficient of 

 friction observed in the case of bodies at rest, i.e., in a condition of static friction, 

 and the co-efficient of friction in the case of moving bodies, i.e., in a condition of 

 kinetic friction ; but Mr. Fleeming Jenkin, in his paper read before the Royal 

 Society in April, 1877, upon the friction between surfaces moving at very low 

 speeds, alludes to the fact that in all cases where a difference in the co-efficient of 

 friction is observed between static and kinetic friction, the static friction exceeds 

 the kinetic. 



Coulomb also points out in his experiments that in the case of static friction 

 the co-efficient of friction increased with the time during which the bodies had been 

 at rest. 



The experiments of Coulomb, Rennie, Morin, and Jenkin were made with bodies 

 moving at comparatively low velocities. 



The following table shows the mean results obtained from a large number of 

 the experiments made with the apparatus above described, upon the action between 

 the cast-iron brake blocks and the wheels fitted with steel tvres : — 



