wind, and as every foot increase in the diameter of the wheel
makes an increase of over 3 ft in the length of the
circumference, the greater the diameter the less the number of
revolutions in any given time; and consequently the kinetic
flywheel action which is so valuable in the smaller sizes is to
a great extent lost in the larger mills.

Any type of pump can be used, but the greatest efficiency will
be obtained by adopting a single acting pump with a short
stroke, thus avoiding the liability, inherent in a long pump
rod, to buckle under compression, and obviating the use of a
large number of guides which absorb a large part of the power
given out by the mill. Although some of the older mills in this
country are of foreign origin, there are several British
manufacturers turning out well-designed and strongly-built
machines in large numbers. Fig. 19 represents the general
appearance and Fig. 20 the details of the type of mill made by
the well-known firm of Duke and Ockenden, of Ferry Wharf,
Littlehampton, Sussex. This firm has erected over 400
windmills, which, after the test of time, have proved
thoroughly efficient. From Fig. 20 it will be seen that the
power applied by the wheel is transmitted through spur and
pinion gearing of 2 1/2 ratio to a crank shaft, the gear wheel
having internal annular teeth of the involute type, giving a
greater number of teeth always in contact than is the case with
external gears. This minimises wear, which is an important matter,
as it is difficult to properly lubricate these appliances, and they
are exposed to and have to work in all sorts of weather.

[Illustration: Fig. l9.--General View of Modern Windmill.]