d there a little pit,
the depolished surface consisting of innumerable hollows of this
description.

But this was not all. By protecting certain portions of the surface,
and exposing others, figures and tracery of any required form could be
etched upon the glass. The figures of open iron-work could be thus
copied; while wire-gauze placed over the glass produced a reticulated
pattern. But it required no such resisting substance as iron to
shelter the glass. The patterns of the finest lace could be thus
reproduced; the delicate filaments of the lace itself offering a
sufficient protection. All these effects have been obtained with a
simple model of the sand-blast devised by my assistant. A fraction of
a minute suffices to etch upon glass a rich and beautiful lace
pattern. Any yielding substance may be employed to protect the glass.
By diffusing the shock of the particle, such substances practically
destroy the local erosive power. The hand can bear, without
inconvenience, a sand-shower which would pulverise glass. Etchings
executed on glass with suitable kinds of ink are accurately worked out
by the sandblast. In fact, within certain limits, the harder the
surface, the greater is the concentration of the shock, and the more
effectual is the erosion. It is not necessary that the sand should be
the harder substance of the two; corundum, for example, is much harder
than quartz; still, quartz-sand can not only depolish, but actually
blow a hole through a plate of corundum. Nay, glass may be depolished
by the impact of fine shot; the grains in this case bruising the
glass, before they have time to flatten and turn their energy into
heat.

And here, in passing, we may tie together one or two apparently
unrelated facts. Supposing you turn on, at the lower part of a house,
a cock which is fed by a pipe from a cistern at the top of the house,
the column of water, from the cistern downwards, is set in motion. By
turning off the cock, this motion is stopped; and, when the turning
off is very sudden, the pipe, if not strong, may be burst by the
internal impact of the water. By distributing the turning of the cock
over half a second of time, the shock and danger of rupture may be
entirely avoided. We have here an example of the concentration of
energy in time. The sand-blast illustrates the concentration of
energy in space. The action of flint and steel is an illustration of
the same principle. The heat required t