PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 21 
gram from the material forming the next briquette for each milli- 
meter variation from 25 in the height of the trial specimen. Six 
briquettes 25 mm. high should be made from each sample. They are 
allowed to dry in the air at ordinary room temperature for 20 hours 
after making, and are then dried at a temperature of 100° C. for four 
hours, cooled for 20 minutes in a desiccator, and immediately tested. 
Briquettes should not be allowed to stand in the open air while 
waiting to be tested, but should be kept in the desiccator. In making 
the test the briquette is held in position on the anvil of the testing 
machine by means of a drop of thick shellac, extreme care being 
taken to see that there is perfect contact between the upper surface 
of the briquette and the plunger, as well as between its lower surface 
and the anvil. There will ordinarily be no difficulty in securing this 
result. In certain highly argillaceous materials, however, the 
shrinkage caused by drying out sometimes warps the briquette so 
that its two ends are not parallel. In such a case a perfect bearing 
can only be secured by very carefully grinding the end of the bri- 
• i 
Failure 62 Blows 
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Fig. 12.— Record of cemonting-value test. 
quette on a piece of fine emery cloth until a satisfactory bearing is 
obtained. With the briquette in position, the plunger resting on 
it, and the cam in the position shown in figure 11, the end of the 
pin (F) is brought into contact with the surface of the cam and the 
locknut tightened. This will give an effective drop of 1 cm. to the 
hammer. The brass pencil is so adjusted that on revolving the drum 
a horizontal line will be traced on the sensitized paper which is 
wrapped tightly around it and held in place by a rubber band. On 
starting the motor, each blow of the hammer will then be recorded 
in a short vertical line above the base line. The number of blows 
required to destroy the resilience of the briquette will equal the number 
of vertical lines showing on the record above the base lines. A typical 
record is shown in figure 12. In counting the number of vertical 
lines showing on the record, no attention should be paid to the fact 
that the short horizontal lines gradually drop below the original base 
line. Each vertical line which shows above the corresponding hori- 
zontal base line on its right should be counted as part of the record. 
The typical record shown in figure 12 brings out this point clearly. 
