12 
perties of rocks a number of examples of unintentional or 
natural contortion have come under my notice. 
]N"ot long ago I saw some small casts of the Elgin marbles 
prepared in the form of long strips of plaster of Paris 21 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 J 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 
horizontally upon corner pedestals. The flagstone of which 
the monuments were constructed had yielded towards the 
unsupported 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 flagstafi" in a public park 
resting upon a broad flagstone 2J 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 the case appeared from the raising of 
the free edges of the slab above the surrounding pavement, 
which was elsewhere fairly level. But all doubt on this 
point was removed by subsequent measurement. A year and 
five months later the centre of the slab had sunk -^-q of an 
inch more, relatively to the ends. 
