Rock Section Cictiiv;/ Apjxiratus. 73 



iDja'hine is inteiL'li;ingeal)le if so required, a rapid transfer from 

 one lap to another can be made. This rorresjtondence between 

 the several parts of the machine, and the facility of inter- 

 change which is thereby effected, result^ in the long- run in a 

 considerable saving of time. 



The most effective laps, so far as my experience goes, are 

 those of bronze containing a high percentage of copper, the 

 aim being to secure a tough but not unduly hard lap. Pure 

 copper laps would no doubt be better but they are difficult to 

 cast and turn. Discs of lead and tin. and also of wood with 

 felted surfaces are used in special cases and for polishing. A 

 diameter of 10 inches is found to l)e convenient for most of 

 these laps. 



Tray-like shields, or mud guards are provided for each of 

 the grinding laps as well as for the slicer. As will be seen, 

 they are of square outline and conveniently large, the distance 

 between the several spindles, 22 inches, permitting of this. 

 The trays are made of stout gah^anised iron 5 inches deep, 

 and the u])per edges are rounded and ])rass bound, forming 

 clean and comfortable supports for the hands and arms of the 

 operator. It should also be noticed that a sjDace around each 

 pillar or spindle is raised and carefully capped, so as to exclude 

 dust and grit ; this, in addition to the brass collars already 

 noticed. The bearings of a machine running at a high speed, 

 and upon wliich car])orundum and other abrasives are to be 

 freely used, cannot be too carefully i>rotected from their in- 

 trusion : the life of the bearings is, in fact, directly propor- 

 tional to the effective exclusion of the abrasives. 



As already stated", the machine is., motor driven, and as the 

 method of connecting up is in some respects novel, I refer to 

 it in some detail. An electrically driven one-horse-power motor 

 serves to rtui the rock slicer, lathe, emery wheel and polisher, 

 and has proved fully adequate for all reqtiirements. As the 

 motor runs at 1400 revolutions per minute, the main shafting, 

 shown in Plate XVI., s))eeded down to about 300, a convenient 

 speed for the drivintr wheels of both lathe and rock slicer. 

 The usual method of gearing to a secondary shaft by means 

 of belts and loose pulleys has been dispensed with, and a 

 system of connecting directly to the main shaft adopted. This 



