Neiv Engine for Riding. 61 



so as to secure as niiich of the fine material as possible. The sedi- 

 ment remaining, consisting mainly of material which settles in ■20 

 minutes, may be put aside for use or further treatment. We may 

 next deal in the same way with the still finer emery contained in the 

 water decanted oft' from the 20 minutes sediment. Much of this will 

 require a still longer time — say 40 minutes — in which to settle and 

 so obtain a further and yet finer grade of emery. The addi- 

 tion of more water and decantation, etc., being continued, 

 aa in previous grades, the time interval only being ex- 

 tended to 40 minutes with a corresponding increase in care 

 in tho several operations. In order to obtain a small supply of 

 the finest effective material present, it will be necessary to carry on 

 the separation process with a 60-minute interval for settlement, the 

 sediment from Avhich will be small in bulk but proportionately 

 valuable for the final abrasive work. 



The process of separating abrasives thus outlined is both slow and 

 tedious, and may extend over one or two days. As, however, reli- 

 able separations are absolutely necessary if good results are desired, 

 it pays to go to considerable trouble to obtain them. 



It is, of course, not necessary for the purpose of grinding one or 

 two screws only to undertake the separation of so large a quantity 

 of material as that above named; but it is essential that an effectual 

 grading of whatever abrasive it is proposed to use be made; and 

 also that the process of separation should be carried to an even 

 greater degree of refinement than is possible with the rough and 

 ready method above outlined. This process only afforded a series 

 of crude powders in which there is considerable variation in the 

 size of the grains. 



To secure greater uniformity of grain, and greater smoothness 

 in working, than is possible with any of the four sediments or 

 separations just described, the following more exact method of treat- 

 ment was adopted : — 



(ii.). — Method of Refinincf crudely separated abrasives. — Before 

 commencing to grind the screws, we had the good fortune to have 

 at hand a series of carefully separated powders of all the abrasives 

 we have described and used; also some knowledge of their behaviour 

 under varying conditions upon a variety of materials. This experi- 

 ence had, in the main, tended to show that the greater the care 

 bestowed upon the separation of an abrasive, the more efficiently 

 and as a rule more quickly, could a desired result be obtained. 



