e
rise and fall of a wave will be found serviceable. A small spur-wheel
is gripped on two sides by two metal laths, with edges serrated like
those of saws, and held against the wheel by gentle pressure. Every
movement of the two saws--whether backwards or forwards--is then
responded to by a continuous circular motion of the wheel, with the
sole exception of those movements which may be too small in extent to
include even as much as a single tooth of the wheel. On this account
it is important that the teeth should be made as numerous as possible
consistently with the amount of pressure which they may have to bear.

Resort may be had to the principle of the aneroid barometer in order
to secure from the water within the pipe-system the energy by which
these saw-like bands are driven up and down with reciprocal motion. A
very shallow circular tank in the shape of a watch is in communication
with the water in the pipes, and its top or covering is composed of a
concentrically-corrugated sheet of finely tempered steel. At the
centre of this is fixed the guide which pushes and pulls the saw-like
laths. Every rise and fall in the pressure of the water now effects a
movement of the spur-wheel, and the latter may conveniently be
connected with the strong spring of a clockwork attachment, so that
the water pressure is really used for winding up a clockwork
ventilating-fan.

In the making of cheap steam and gas engines, as well as in machine
work generally, rapid progress will be made when the possibilities of
producing hard and smooth wearing surfaces without the need for
cutting and filing rough-cast metal have been fully investigated. Many
parts of machinery will be electro-deposited--like the small articles
already mentioned--in aluminium or hard copper at the metallurgical
works where ore is being treated for the recovery of metal, or even at
the mines themselves.

Side by side with this movement there will be one for developing the
system of stamping mild steel and then tempering it. At the same time
also the behaviour of various metals and alloys, not only in the cold
state but also at the critical point between melting and
solidification, will be much more carefully studied so as to take
advantage of every means whereby accurately shaped articles may be
made and finished in the casting. It has been found, for example, that
certain kinds of type metal, if placed under very heavy pressure at
the moment when passing from th