9 
•01 in. If in Mjnsting the Imife-edge tlie index is dis- 
placed, it can be restored by tbis 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 *! to 
•05 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 deflection 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 extending 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 appeared.* It will 
now suffice to say that thin plates of mountain limestone 
(especially a certain bituminous kind, occurring in thin beds 
with partings of shale) proved indeflnitely 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 foimd 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 
* ''Britiah Association Eeport," 1869 ; "Geological Magazine," November, 1869. 
