Nevj Engine for Riding. 83 



being extremely fine diamond separated in oil. If the polishing 

 •operation is properly carried out it leaves the surface of the crystal 

 •optically true, scratchless and with a brilliant polish. The outer 

 zone of the rectangular face of the crystal was next turned off in the 

 lathe, ^^itll the aid of a diamond tool, leaving the central area cir- 

 cular in outline and slightly above the surrounding face (see I'ig. 

 1, Plate XVI.). The task of grinding and polishing the plate was 

 throughout controlled and corrected from time to time, as required, 

 with the aid of a small test plane, in order to ensure the best possible 

 result. Any trace of error, when tested in this way, should nowhere 

 exceed some small fraction of a wave length. This condition is 

 attainable with time, jDatience and a moderate amount of manipula- 

 tive skill. 



It remained now to provide for the Mounfing and Adjustment 

 of the thrust plate prepared as described. 



With respect to the former, viz., Mounting. The plan adopted 

 for mounting the prepared thrust plate was as follows : — A block 

 of machined cast iron was first prepared. This in elevation was tri- 

 angular in outline Avith rectangular base and perpendicular face. 

 The base was 1^ in. square, the perpendicular face 2 in. high. 

 Plate VI. shows the position of the thrust in relation to surrounding 

 parts of the machine; also the method of its attachment to the bed. 

 Fig. 1, Plate XVI., is a photograph, slightly enlarged portion of the 

 perpendicular face of the block, the upper part of which shows the 

 method of attaching the crystal plate to its bed. This attachment 

 was effected with the aid of a small rectangular frame the inner 

 ■edges of which were undercut to fit the bevel upon the edges of the 

 ■crystal plate. The latter was then inserted into the frame in which 

 it was embedded with a suitable cement composed mainly of shellac. 

 By this means the thrust plate is made both secure and rigid within 

 its frame without undue strain, while at the same time its face is 

 adjusted approximately perpendicular to the base of its supi>ort. 



Final adjustment of the thrust plate and sereio thrust. — As tliere 

 is a mutual relationship between the sapphire thrust plate and the 

 screw thrust, their respective adjustments may be treated concur- 

 rently. 



The shape of the screw thrust finally adopted was that of a 

 blunt cone of hard steel with a small flat termination, the diameter 

 of the latter being about .1 in. This form of thrust, up to the 

 present, has behaved satisfactorily and promises to prove per- 

 manent, as it has now been in use for a considerable time. 



