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POPULAR SCIENCE REVIEW. 
applied to the edges of the plates of rock, and after a few 
failures the pitch did its work sufficiently well. I regard the 
matrix of pitch as almost indispensable to success when the 
experiment of lateral pressure is performed. 
By the process just described slabs 9 in. long have been 
bent until they rose -§ in. in the centre. More conspicuous 
results may be expected hereafter. The operations required 
for the production of apparently inconsiderable deflection are 
tedious and slow, but patience rather than fresh methods seems 
to be needed. The contortions which we would imitate were 
not made in a day. Nature is as superior to us in resources of 
time as of power. Completer and more varied experiments 
than these are to be desired. Larger specimens of rock should 
be tested, and the exposure should be longer than thin plates 
require. With appropriate apparatus a series of observations 
as detailed and exact as those instituted by Fairbairn in the 
case of iron and steel might be carried out, greatly to the ad- 
vantage of geologists, physicists and engineers. 
While occupied with this subject of the mechanical proper- 
ties of rocks a number of examples of unintentional or natural 
contortion have come under my notice. 
Not long ago I saw some small casts of the Elgin marbles 
prepared in the form of long strips of plaster of Paris 2J in. 
broad. These casts had been laid aside for some years and 
had warped visibly. In one case the deflection (estimated as 
a rectilinear angle) amounted to 6°. This led to some experi- 
ments on plaster of Paris. On submitting dry plates \ of an 
inch in thickness to the knife-edge machine a deflection of 8° 
was obtained in six weeks, and I soon found that this material 
is indefinitely plastic if the strain be gradually applied. It 
would probably be easier to bend a flat plate of plaster of Paris 
into a cylinder than a plank of deal. 
Walking one rainy day past the burying-ground attached to 
a country chapel, I found some gravestones supported horizon- 
tally upon corner pedestals. The flagstone of which the 
monuments were constructed had yielded towards the unsup- 
ported centres, and there were pools of water standing in the 
hollows. The sculpturing of the inscriptions was too sharp to 
admit the supposition of extensive weathering. The stones 
were quite smooth, and the method of rubbing down the surface 
must have rendered them quite level before erection. 
Shortly afterwards, I saw a flagstaff in a public park resting 
upon a broad flagstone 2 J inches in thickness and supported in 
an upright position by iron ties fixed in the ground at a short 
distance. The weight of the mast I guessed to be about two 
tons. The flagstone at its base was visibly curved, as if it had 
bent beneath the weight of the pole. That this was actually 
