tered upon,--in fact, the "little want" of which Lord
Kelvin spoke in 1881 will have been supplied. The high speed which the
dynamo requires, and the more rapid rate at which windmills
constructed on this very economical principle must necessarily run,
both mark the two classes of apparatus as being eminently suited for
mutual assistance in future usefulness.

The anemometer of the "Robinson" type, having four little
hemispherical cups revolving horizontally, furnishes the first hint of
another principle of construction adapted to the generation of
electricity. Some years ago a professor in one of the Scottish
Universities set up a windmill which was simply an amplified
anemometer, and connected it with several of Faure's storage batteries
for the purpose of furnishing the electric light to his residence. His
report regarding his experience with this arrangement showed that the
results of the system were quite satisfactory.

In this particular type of natural motor the wind-wheel, of course, is
permanently set to run no matter from what direction the wind may be
blowing. Tests instituted with the object of determining the pressure
which the wind exerts on the cup of a "Robinson" anemometer have shown
that when the breeze blows into the concave side of the cup, its
effect is rather more than three times as strong as when it blows
against the convex side. At any given time the principal part of the
work done by a windmill constructed on this principle is being carried
out by one cup which has its concave side presented to the wind,
while, opposite to it, there is another cup travelling in the opposite
direction to that of the wind but having its convex side opposed.

The facts that practically only one sail of the mill is operative at
any given time, and that even the work which is done by this must be
diminished by nearly one-third owing to the opposing "pull" of the cup
at the opposite side, no doubt must detract from the merits of such a
wind-motor, judged simply on the basis of actual area of sail
employed. But when the matter of cost alone is taken as the standard,
the advantages are much more evenly balanced than they might at first
sight seem to be.

The cup-shaped sail may be greatly improved upon for power-generating
purposes by adopting a sail having a section not semicircular but
triangular in shape, and by extending its length in the vertical
direction to a very considerable extent. Practically this cheap