64 H. J. Graysov : 



to point out that it is extremely difficult to secure uniformity in the 

 distribution of powders so tine in grain as those represented owing 

 to their tendency to clot or segregate; and also that the magnifica- 

 tion used was somewhat too high for the coarse-grained samples, 

 and too low for the finer ones. In spite of these drawbacks, the 

 photographs render sufficiently apparent the differences in size and 

 character of the grains. For example. Figs. 1 and 2, Plate XII., 

 illustrating alundum, the grains of which are respectively .0015 in. 

 and .0003 of an inch, are adequate to make clear the difference in 

 grain between the respective sizes. Again, Figs. 3 and 4, 

 representin<j; two examples of carborundum, show clearly the 

 more angular character of the grains as contrasted with alundum, 

 and a coarse emery. The difference between carborundum and 

 emery, Avith the same sized grain, is less striking but yet appre- 

 ciable; the emery being the more uniform of the two samples. 



Fig. 5, PI. XII., sho^vs an emery with an average grain of .001 in. 

 diam., the coai'sest abrasive used upon the screw, and with the 

 rounded character of grain already referred to. The fine emery 

 shown as Fig. 6 was used mainly for finishing purposes only, 

 acting rather as a polishing agent than- as an abrasive, and leaving 

 the screw tlireads in a semi-polished condition. 



Combinations of emery, alundum and carborundum, having the 

 same size of grain, were occasionally used, and proved advantageous 

 in the earlier stages of the grinding process. 



Having prepared a series of abrasives, the oi^eration of grinding 

 the screws was entered upon, and may for convenience be divided 

 into two stages, the first of these being the prelitninary rough grind- 

 ing by hand, using a relatively coarse abrasive, and the second 

 stage, grinding with fine abrasives and more or less comjjlefe 

 automatic or mechanical control. 



(iii.). — The Freliminary grinding of the acrew hy hand. — This 

 operation involved fitting up the lathe with extra long centres so as 

 to permit the grinding nut to travel beyond the threaded part of the 

 screw bars for at least several inches at each end. For this pur- 

 pose, the screw was revolved slowly, while the nut was held and 

 controlled by hand. The whole of the grinding at this stage was 

 done with one grade of emery only, viz., that described at Fig. 5, 

 PI. XII., having an average grain diameter of .001 inch. This 

 emery, mixed with oil, was fed into the nut along the slot, the 

 ring clamps being released to permit of its introduction and even 

 distribution. Pressure upon the nut was applied very gradually 



