New Engine for Riding. 85 



•either the front or back surface of the block base as required. The 

 -effect of every adjustment was determined -with the telescope and 

 waa followed by such further alterations as was indicated by the 

 motion of the image of the cross wires. The process of correction 

 was continued in this way until the telescopic image remained steady 

 within the limits of experimental error. Finally the mirror was 

 removed from the rod representing the screw and was placed upnn 

 the actual screw of the machine and a slight further adjustment of 

 the thrust plate made by the same method. 



As the thrust block could now be removed and restored to its 

 position tetween the micrometer screws with certainty, it was taken 

 out and the surface of the sapphire coated with a very thin film of a 

 solution of eosin in alcohol and water, applied with the aid of a 

 fine smooth needle. In this way an immeasurably thin film of stain 

 or colour was left upon the polished sapphire face. On replacing 

 the block in its former position the thrust facet of the screw was 

 ;gently pressed into contact with the sapphire and rotated through 

 a very small angle. The effect produced was carefully examined 

 with a powerful magnifier and the exact position and area of contact 

 determined; this showed that the surface of contact was not central 

 but was limited to one part of the thrust near its edge. Another 

 test with the screw I8O0 from its former position definitely indicated 

 that the screw facet was principally at fault. Correction of this 

 defect was obtained by inserting a thin parallel plate of quartz 

 between the thrust plate and screw bearing; one face of the quartz 

 was polished the other smoothly ground, the latter being in contact 

 with the steel thrust. The screw was then slowly rotated in contact 

 with the plate which was constantly moved, i-evolved and reversed 

 between the thrusts. The result of these combined movements was 

 a delicate abrasive action upon the screw facet without any cor- 

 responding effect upon the sapphire thrust face. In this way, with 

 <;arQ and judgment, fairly uniform central contact of the screw 

 thrust face with that of the sapphire was obtained. It should be 

 understood that the amount of material removed from tHe bearing 

 surface of the thrust by this process was probably not more than one 

 or two wave lengths in thickness. These correctional operations, 

 which extended over a considerable period, with intervening trial 

 rulings, resulted eventually in the almost complete elimination of 

 any remaining periodicity. 



