186 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
mitted to bear as a load upon the plunger, and through that upon the cube. This gave 
an initial pressure (which was varied according as the species of stone was harder or 
softer) of a few hundred pounds, usually about at the rate of 500 lbs. per square inch, by 
which all parts of the testing-lever and the crushing faces with the cube were brought 
into perfect contact and bearing. 
113. The dimensions of the cnbe having been accurately measured and noted before 
putting it into the crushing-cage, the altitude of the cube, or rather the exact distance 
between the crushing faces, was then taken. From the thinness of the papers on the 
top and bottom faces and the compression of these, this distance did not differ from the 
true vertical height of the cube by g-jjo of an inch. 
114. Water was now admitted in a small stream to the load bucket ; and as each 1000 
pounds of load was attained, the flow was stopped and the amount of compression of the 
cube taken by measurement between the crushing faces, as already described. The 
loading was thus continued until the first symptoms of disintegration were noticed in 
the cube, almost always showing itself by one or more extremely minute cracks in nearly 
vertical planes, or by a few fragments of powder detaching themselves from the sides. 
At this stage the loading was stopped and the compression noted. The loading was 
then continued slowly until the cube finally gave way, crushing down before the descending 
plunger into absolute powder, which remained impacted together as a flat cake beneath 
the plunger, hindering its further descent. 
115. The vertical distance between the face of the plunger and that of the base-plate 
(fig. 9) was then accurately taken, and this, deducted 
from the initial distance, gave the absolute distance 
through which the plunger had descended. 
116. Had the entire volume of material in the cube 
remained always beneath the plunger in the form of a 
short cylindrical or round flat cake of powder impacted 
together and of equal density with the cube before it 
was crushed, inasmuch as the diameter of the plunger 
was 3 5 inches, the vertical distance a b (fig. 9) would 
have been for an exact cube of T5 inch = 0-351 inch, 
which, deducted from 1-5 inch, would have given the descent of the plunger. 
117. But the cubes were not all quite equal in volume, and the whole of the powder pro- 
duced did not always remain impacted under the plunger, but some was thrown to a little 
distance. Hence it was necessary to measure in each case, to obtain the actual descent. 
In a few cases, with the more rigid and elastic rocks, just before the final crush down, 
the cube split off one or more comparatively large undisintegrated fragments, which 
were thrown out laterally and escaped further action. Whenever any such fragments 
occurred means were adopted for their collection and preservation ; and in such cases a 
correction has been applied to the result of the crushing based on this allowable assump- 
tion, that the work expended upon crushing that portion of the entire cube which was 
Fig. 9. 
Y 
* K. 
a 
cube 
b 
u~ 
1 
-4. A- - 
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