Neiv Engine for Riding. 65 



at this stage, and the grinding continued for ten minutes, when the 

 nut was reversed and a fresh supply of emery fed in. Thus the 

 work of grinding went on under moderate pressure, regular 

 reversal of the screw and fresh supplies of emery and oil. 



As already mentioned, two screws were ground with the same nut, 

 which necessitated a change from one to the other about every half- 

 hour. No very great precautions were taken to maintain uni- 

 formity of temperature during these early operations, nor did this 

 appear to be necessary, seeing that each grinding operation lasted 

 only for a comparatively short time. The grinding, as described 

 above, covered about 12 hours' actual work, which was distributed 

 over several days. During this preliminary grinding, attention 

 was mainly given to the supply of abrasive and oil, which was 

 carefully and uniformly distributed over the full length of both 

 screw and grinding nut at frequent intervals, the precaution being 

 taken not to exhaust its action before renewal. Cai'e was taken to 

 apply only moderate and uniform pressure upon the nut, which 

 was regularly reversed and changed with respect to the screw, and 

 supported so as to avoid flexure. The precaution to wash out the 

 nut with kerosene, and clean ofp the screw with cotton waste about 

 every three hours, was not overlooked. 



When inspection with a magnifying lens indicated that the whole 

 of the threads appeared to have been ground to a uniform con- 

 dition, as shown by the disappearance of any tool markings, it 

 became necessary to devise some simple form of test, capable of 

 revealing any very marked periodicity, and general condition of the 

 screw throughout its entire length as regards the crowns, roots and 

 angles of the threads and their bearing surfaces. For the rapid 

 inspection of the screw with respect to these features, a vsmall 

 examination bench was constructed as follows : — A platform of 

 thick plate glass, rather longer than the screws and three inches 

 wide, was supported at each end, at a height of about three inches 

 above a wooden base. Screwed to this base were two guides, also 

 of wood, adjusted so as to permit the square base of a microscope 

 stand to slide between them from end to end of the platform. 

 During an examination the screws rested upon the glass platform 

 with the threads interlocked and adjusted parallel to the travel of 

 the microscope along the main base. With the aid of a long strip 

 of white card, placed below the platform supporting the screws and 

 adjusted to an angle suited to the direction of the incident light, 

 ample illumination for a magnification of 20 diameters or more 



