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POPULAR SCIENCE REVIEW. 
descends a vertical plate c , terminating in a hinged knife-edge 
e. We get in this way pressure applied always to the same line 
upon the lamina of rock, for when deflection begins the knife- 
edge inclines forwards out of the perpendicular at its lower 
edge without sliding over the rock specimen. By pushing the 
block / along the groove h perpendicularity is restored. An 
index m connected with e makes any deviation more apparent. 
When a piece of rock is to be tested, shot is poured into the 
cylinder ct, which is in direct communication with the vertical 
plate c, and the pressure is taken by a steelyard or balance. 
The index l is set at zero by the screw i, and its motion along 
the graduated scale enables the observer to record with pre- 
cision a deflection of less than *01 in. If, in adjusting the 
knife-edge, the index is displaced, it can be restored by this 
screw independently of other parts of the machine. 
With this apparatus I began a long series of observations on 
limestone. Thin plates of various thickness from T to *0 5 in. 
were subjected to low but protracted pressures. Experience 
taught the best form of plate and the time required to produce 
a given result. I succeeded in one case in bending a plate 
•07 in. in thickness to an angle (reckoned as rectilinear) of 12°. 
This took three months to accomplish. On removal from the 
machine the plate cracked near the apex of the angle of de- 
flection in three days, or I should have operated upon it again. 
The pressure was applied so gently and uniformly that sudden 
fracture seldom occurred except when intentionally produced. 
The bent slabs were, however, very fragile, and could seldom be 
kept many days after released from strain, cracks slowly ex- 
tending themselves transversely across the part where the 
deflection was greatest. From this circumstance, which caused 
much annoyance at the time, some useful lessons were learned. 
Details of the experiments with limestone have already ap- 
peared.* It will now suffice to say that thin plates of moun- 
tain limestone (especially a certain bituminous kind, occurring 
in thin beds with partings of shale) proved indefinitely plastic. 
The elasticity of the rock was greater than I had expected, but 
the set or permanent deflection produced by long-continued 
pressures of inconsiderable amount far exceeded what I had 
hoped to find. It may be doubted whether there is any limit 
to the bending which a careful and patient observer can pro- 
duce. I found that magnesian limestones, while usually much 
more elastic than specimens of pure carbonate of lime, were 
slightly more plastic. The two properties are not connected in 
any direct or inverse ratio that I can discover. Some of the 
* u British Association Report/’ 1869 ; “ Geological Magazine/’ Novem- 
ber, 1869. 
