494 



EEPORT — 1886. 



Experiments with Special Apparatus. 



In designing tlie sphere and roller machines it was assumed that 

 between the rolling surfaces the coefficient of friction was 



/i=0-l, 



and consequently that the pressure required between the ball and the 

 disc must be ten times as great as the force to be transmitted. During 

 the experiments above recorded it was found that the 'coefficient of fric- 

 tion was not constant, but increased with the pressures, the highest value 

 being about 



/i=0-16. 



The machine at the Liverpool Exhibition gave results even higher 

 than this, and, generally speaking, it has been observed that the value of 

 the coefficient at the high pressures used increases slightly with the pres- 

 sure. It is evident that the circumstances of the case are peculiar, and 

 that even the few published experiments npon rolling friction — such as 

 those upon locomotive tyres — did not furnish satisfactory data for guid- 

 ance. It was therefore determined to conduct a series of experiments with 

 special apparatus, and for this jjurpose the machine shown in figs. 11 and 12 



Figs. 11 & 12. 



was designed. This apparatus consists of two discs (a) and (6), six inches 

 in diameter, the edges of which are in contact, and which can be replaced by 

 other similar discs of diiferent material. One of these discs (a) is driven 



