10 BULLETIN 347, U. S. DEPARTMENT OF AGRICULTURE. 
of the bevel gear (D), driven by the pulley (C). At F is shown a 
brass grip for holding the test specimen. The grip carrying the speci- 
men is held in a vertical position over the steel disk by inserting it 
in the sleeve (G), which is of such size as to form a sliding fit allowing 
free vertical but no sidewise movement of the grip. The abrasive, 
crushed quartz is fed onto the disk through the funnel (H), the lower 
end of which should form an opening one-fourth of an inch in diameter. 
The sleeve (G) is so placed that the center of the grip, when in position, 
will be 26 cm. from the center of rotation of the disk. A revolution 
counter is mounted as at (B). A galvanized-iron hopper, not shown, 
is fitted under the disk to receive and carry away the used sand. 
Crushed quartz sand. 
The sand used as the abrasive agent is a crushed quartz, screened 
to pass a standard sieve having 30 meshes per linear inch and to be 
retained on a standard sieve having 40 meshes per linear inch. Since 
it is almost impossible to obtain such a sand commercially, it is 
customary to specify a sand not more than 5 per cent of which will 
be retained on a No. 30 sieve and not more than 25 per cent of which 
will pass a No. 40 sieve. Sand known to the trade as No. 2| quartz 
will usually fulfill these requirements. 
Standard sieve 16 inches in diameter, with 30 meshes per linear 
inch. 
Standard sieve 16 inches in diameter, with 40 meshes per linear 
inch. 
Balance, same as used in abrasion test. 
Several small metal washers. 
A core 25 mm. in diameter and about 10 cm. long is cut with the 
diamond drill from the specimen to be examined. To use the 
diamond drill properly, so as to avoid undue wear, requires great care 
and considerable experience. It is convenient to mount the specimen 
under test on the table of the drill press by means of a bag filled with 
sand, which forms a cushion for the irregular, projecting edges of the 
rock. Very small specimens may be mounted in plaster of Paris. 
The core should in every case be drilled perpendicular to the bedding 
plane of the rock, using the hand feed of the drill press only, with 
just enough pressure to cut. Should the core be not cut entirely 
through the specimen it may be removed by inserting the end of a 
small screw driver into the cut and using it as a lever to break the 
core away from the bottom. After thoroughly drying, the specimen 
is inserted in the brass grip of the Dorry machine, leaving about 1 
inch projecting from the lower end. The grip is then inserted in the 
sleeve so that the lower end of the specimen rests on the steel disk. 
The funnel is filled with sand and the machine run until the lower end 
of the specimen has been worn down to the plane of the disk. The 
