Neiv Engine for Riding. 89 



laroken into several pieces a gentle blow will serve to reduce the 

 larger fragments to a suitable size for ruling purposes. The broken 

 splinters are placed in a watch glass and roughly sorted from the 

 small debris. The selected pieces are then carefully examined under 

 a higher magnification and those showing good knife edges and 

 •cleavage faces intersecting a smooth outer crystalline face, may be 

 selected for trial. After a sufficient number have been thus 

 obtained they are mounted in cup-shaped depressions drilled on 

 the ends of short pieces of straight hard brass of the correct gauge 

 to fit the holder of the m.achine. The depressions in the rods are 

 partly filled with a hard tough cement rendered plastic by heating. 

 The diamonds are j^laced in position upon the cement which is 

 gently heated and moulded around the diamond fragment until the 

 latter is deemed to be secure. The whole of this work is done with 

 the aid of a microscope and requires both care and experience. A 

 popular notion concerning diamonds is that they are so excessively 

 hard that they may l>e handled with impunity. This notion is 

 speedily falsified upon a very short experience of the fragile edge of 

 a diamond suited for ruling lines at the rate of 20,000 to the 

 inch. 



When a few promising fragments have been cemented in their 

 holders they are placed in a small ruling machine, and tested with 

 i-espect to correct centring and inclination or angle of the knife's 

 edge to the surface being ruled. The setting of a diamond for 

 ruling is almost the reverse of that required when it is used as a 

 turning or cutting tool upon a lathe. When used for the latter 

 -purpose, it is required of a diamond to definitely remove materials 

 from a given surface. As a ruling instrument the diamond usually 

 ■only slightly displaces or compresses material according to the 

 nature of the material. The lines, for example, represented in the 

 photographs, Plate XVII., are nearly all examples of slight dis- 

 placement or compression. Moreover the diamond is not rigidly 

 held during the act of ruling but is trailed along the surface 

 operated on. 



The pressui-e exerted upon the cutting edge of a diamond must 

 be carefully proportioned to the length of its contact with the sur- 

 face ruled, and this does not usually in the case of fine lines exceed 

 .002 in. The angle the cutting edge makes with the surface ruled 

 upon must at the same time be taken into account, as this angle 

 has an intimate relation with the length of contact on the ruled 

 surface and hence with the pressure required for a satisfactory line. 



