50 H. J. Grayson : 



must througlKHit be without appreciable change in depth, -width, or 

 in its capacity to reflect light. These and other considerations, 

 which are particularly well described by Professor Michelson,^ aff"ord 

 some idea of the amount of care required in the selection, testing 

 and adjustment of a ruling diamond. As it is our intention to 

 refer in detail to the selection, preparation and general manipula- 

 tion of a diamond suitable for ruling fine lines, and to illustrate 

 its action under varying conditions, further reference is, for the 

 present, postponed. 



Driving Mechanism. 



This includes chiefly the driving Engine, also such parts of the 

 machine as are essential for the transmission of motion to those of 

 its features just described. Respecting the motive power : while 

 the total energy required may be very small — say below J H.P., it 

 must be continuously and uniformly exerted over long periods. Any 

 stoppage, no matter for how short a time, would be ruinous in its 

 effect. Even a slowing down of the engine, such as would cause a 

 variation in speed of more than a few j^er cent., would be highly 

 detrimental. 



It appears to have Ix^n the usual practice to select some type of 

 water or electric motor for operating the most recently constructed 

 ruling engines. Electric powder seems to have been availed of by 

 Rowland, and also for driving the Blytheswood machine, now at the 

 National Physical Laboratory. 



No doubt the electrical drive may be advantageous when suitable 

 storage batteries are exclusively available. As access to such bat- 

 teries Avas not at my command it was decided to try the next most 

 suitable driving-power obtainable, viz., a small hot-air Engine. 

 Several preliminary trials Avith this type of motor were very pro- 

 mising, and as the total energy required was found to be very 

 small, not more than 1-40 H. P., an engine of that nominal capacity 

 proved to be fully adequate for all requirements. 



This motor has now been in almost continuous use for about three 

 years and has never once failed. It runs, with a minimum of atten- 

 tion, for comparatively long intervals. And as it gives off but 

 little waste heat, and is free from vibration and noise, it can be 

 placed in a room adjoining that of the ruling engine. It has, 

 in fact, proved an almost ideal engine for accomplishing a some- 

 what difficult task. 



1 See "Nature." Jan. Hth, 1912. 



