and
efficient wind-motor then becomes a square or hexagonal upright axis
of fairly large section, to each side of which is secured a board or a
rigid sheet-metal sail projecting beyond the corners. The side of the
axis and the projecting portion of the sail then together form the
triangular section required.

For the sake of safety in time of storm, an opening may be left at the
apex of the angle which is closed by a door kept shut through the
tension of a spring. When the wind rises to such a speed as to
overbalance the force of the spring each door opens and lets the blast
pass through. One collateral advantage of this type of windmill is
that it may be made to act virtually as its own stand, the only
necessity in its erection being that it should have a collar fitting
round the topmost bearing, which collar is fastened by four strong
steel ropes to stakes securely set in the ground. The dynamo is then
placed at the lower bearing and protected from the weather by a metal
shield through which the shaft of the axis passes.

For pumping, and for other simple purposes apart from the use of
the dynamo, a ready application of this form of wind-engine with
a minimum of intricacy or expense may be worked out by setting the
lower bearing in a round tank of water kept in circular motion by
a set of small paddles working horizontally. Into the water a
vertically-working paddle-wheel dips, carrying on its shaft a crank
which directly drives the pump. This simple wind-motor is particularly
safe in a storm, because on attaining a high speed it merely "smashes"
the water in the tank.

Solar heat is one of the principal sources of the energy to be derived
from the wind. Several very determined and ingenious attempts at the
utilisation of the heat of sunshine for the driving of a motor have
been made during the past century. As a solution of a mechanical and
physical puzzle, the arrangement of a large reflector, with a small
steam-boiler at the focus of the heat rays thrown by it, is full of
interest. Yet, when a man like the late John Ericsson, who did so much
to improve the caloric engine, and the steam-ship as applied to
war-like purposes, meets with failure in the attempt to carry such an
idea to a commercially successful issue, there is at least _prima
facie_ evidence of some obstacle which places the proposed machine at
a disadvantage in competition with its rivals.

The solar engine, if generally introduced, would be found