84 H. J. Grayson : 



Although it may not Ije absolutely essential that both thrust faces 

 sliould be strictly parallel and perpendicular to the screw axis, one of 

 them must be exactly so placed and the other as nearly so as pos- 

 sible; hence the adjustment of both was effected as part of the same 

 operation. 



In the first place, the cone and flat of the inserted screw thrust 

 was ground Avith all due precaution in the lathe and a surface 

 approximately perpendicular to the axis of the screw Avas obtained. 

 This, however, was not accurate enough for the purpose required 

 and, as the further adjustment of the end of the screw was related 

 to that of the sapphire face, the method adopted for their mutual 

 correction will be exj^lained with the setting up of the thrust block. 

 In order to obtain a close approxiniation to its true position the 

 sapphire thrust block was first set up with the aid of the usual mecha- 

 nical appliances. More exact adjustment was obtained by the use of 

 a well-known optical method of setting up a mirror normal to a rota- 

 ting axis. A round bar of steel, rather longer than the screw, was 

 accurately ground at each end to the same diameter as the screw 

 bearings. This bar formed the counterpai't of the screw without 

 its threads, and hence could be used with greater freedom and with- 

 out risk. To one end of the bar, representing the thrust end of the- 

 screw, a truly plane quartz mirror was attached by means of a 

 slightly plastic cement. This rod was placed in the bearings of the 

 ruling machine, and the mirror firmly pressed without rotation 

 against the sapphire thrust face. The rod was then transferred tO' 

 V bearings so placed that by means of a reading telescope of fairly 

 high power a distant illuminated disc Avith cross wires and perpen- 

 dicular scales could be viewed by means of the quartz mirror. 

 Rotating the rod caused the image of the cross wires to move in the 

 field of view unless the surface of the mirror Avas normal to the axis 

 of the rod. Definite movements corresponded, as was soon found, 

 to definite errors in the thrust and so by means of trial and error 

 the thrust Avas, after considerable AA'ork, adjusted until the image 

 of the cross wires remained fixed during a rotation. As the sapphire 

 thrust had already- l>een adjusted to a nearly correct position, these 

 later adjustments Avere only of microscopic dimensions. For adjust- 

 ment in a horizontal plane, the small movements required were 

 effected with the aid of tAA-o micrometer scrcAvs placed one on the 

 right and the other on the left of the thriist block to which they 

 could be made to impart a minute rotary movement. Adjustment of 

 the block in a vertical direction Avas obtained by slightly grinding 



