11 
in this second aeries of experiments. It is easy to see that 
when deflection begins the bending force is increased in a 
high ratio. We have the pressure acting upon the slab, not 
as a force transmitted through its plane, but concentrated 
upon the centre, the two halves acting as levers. As the 
contortion proceeds the strain increases rapidly, and in prac- 
tice it is found that no graduated pressure can be contrived 
sufficiently delicate to avoid sudden fracture. 
To overcome such difficulties as these I imbedded thin 
slabs of limestone in pitch and fitted them into a cast-iron 
box, the two sides of which were removed. One end was 
cut to allow a screw to travel through it, and within was a 
plate of iron which could be moved along by the pressure of 
the screw so as to tighten the slab within the box. By this 
means pressure was 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 f 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 advantage of geologists, physicists, and 
engineers. 
While occupied with this subject of the mechanical pro^ 
