[13] 



DIRECTIONS FOR COLLECTING ROCKS MERRILL. 



length, of the fold instead of merely at the bottom. It will very likely strike the 

 reader that a better material than paper might be found. I can only state that after 

 considerable experimenting the paper was, all things considered, found most satis- 

 factory. 



The work of grinding tlie section, altliough. laborious, requires only 

 sucli skill as almost anyone can shortly acquire. Where but few are 

 to be i)repared the grinding may be done entirely by hand, and on any 

 smooth piece of cast iron, such as the circular covers used on cooking 

 stoves and ranges. This process is necessarily slow, and .whenever 

 possible it is best to have a simple machine consisting of one or more 

 horizontally revolving iron plates some ten to twelve inches in diameter, 

 driven by foot or other power. This plate, or lap as it is not infre- 

 quently called, should be made to revolve in a zinc or lead lined box^ 

 the sides of which are elevated an 

 inch or so above the level of the 

 plate. This not merely to prevent- 

 the mud from the wheel from flying- 

 over the workman, but for the iDur- 

 pose of preserving the mud, which 

 may be used over many times to 

 advantage. It is essential that the 

 lap be driven by means of a belt 

 rather than cogwheels, and that 

 the shaft have a balance wheel 

 sufficiently large to give a uniform 

 rate of revolution. How fast the 

 plate should revolve depends on 

 the individual. If too rapidly the 

 emery flies off without doing its 

 work. About 1,000 revolutions to 

 the minute has been found quite 

 satisfactory in this department. 

 Such a machine may be made at small cost by any intelligent machinist 

 (see fig. 17). The demand has as yet been scarcely sufficient to cause 

 them to be kept ready-made. In the Johns Hopkins University Cir- 

 culars (Yol. XII, No. 103, 1893), and also in the American Journal of 

 Science for February, 1893, was described and figured a machine built 

 under the direction of the late Dr. George H. Williams. 



When the nature of the material is such as to necessitate sawing-^ 

 instead of breaking with the hammer, in order to get the desired chip 

 for grinding, a horizontally revolving tin or soft iron plate some 10 

 inches in diameter may be used, the cutting material being fine emery 

 or carborundum mud applied with a brush in one hand while the object 

 to be cut is pressed against the edge of the plate with the other. A 

 better though from necessity more expensive method is afforded by the 

 small diamond saws made by W. 0. Kerr, of 292 Westminster street. 

 Providence, E. I., at $12 ijer dozen. These are simply tin disks charged 



Fig. 17.— Grinding wheel. 



