and piston being arranged as described,
may now be held by an assistant and the demonstrator, taking a
sledge hammer, may proceed to strike steadily on the end of the
piston and, although the paper will bulge out a little, the force
of the blow will not break it.

"If the assistant holding the tube allows it to jerk or rebound
after each blow of the hammer, the paper may break, because air and
sand are driven down by the succeeding blow, and therefore it must
be held steadily so that the piston bears fairly on the sand each
time.

"A still more conclusive and striking experiment may be shown with
a framework of metal constructed to represent a pail, the sides of
which are closed up by pasting sheets of tissue paper inside and
over the lower part. As before demonstrated, when a quantity of
sand is poured into the pail the tissue paper casing at the bottom
does not break, but if a sufficient quantity is used the sides
formed of tissue paper bulge out and usually give way in
consequence of the lateral pressure exerted by the particles of
sand."

The writer has made the second experiment noted, with special apparatus,
and finds that with tissue paper over the bottom of a 2-in. pipe, 15 in.
long, about 12 in. of sand will stand the blow of a heavy sledge hammer,
transmitted through a wooden piston, at least once and sometimes two or
three times, while heavy blows given with a lighter hammer have no
effect at all. That this is not due in any large measure to inertia can
be shown by the fact that more than 200 lb. can safely be put on top of
the wooden piston. It cannot be accounted for entirely by the friction,
as the removal of the paper allows the sand to drop in a mass. The
explanation is that the pressure is transmitted laterally to the sides,
and as the friction is directly proportional to the pressure, the load
or effect of the blow is carried by the proportional increase in the
friction, and any diaphragm which will carry the direct bottom load will
not have its stresses largely increased by any greater loading on top.

The writer believes that experiments will show that in a sand-jack the
tendency will be for the sides to burst rather than the bottom, and that
the outflow from an orifice at or near the bottom is not either greatly
retarded or accelerated by ordinary pressure on top. The occurrence of
abnormal voids, however,