as a new one.

[Illustration: A typical vertical turbine]

Reaction turbines may be either horizontal or vertical. If they are
vertical, it is necessary to connect them to the main shaft by means
of a set of bevel gears. These gears should be substantially large,
and if the teeth are of hard wood (set in such a manner that they can
be replaced when worn) they will be found more satisfactory than if of
cast or cut metal.

[Illustration: Two wheels on a horizontal shaft

(Courtesy of the C. P. Bradway Company, West Stafford, Conn.)]

The horizontal turbine is keyed to its shaft, like the impulse wheel,
so that the wheel shaft itself is used for driving, without gears or a
quarter-turn belt. (The latter is to be avoided, wherever possible.)
There are many forms of horizontal turbines; they are to be had of the
duplex type, that is, two wheels on one shaft. These are arranged so
that either wheel may be run separately, or both together, thus
permitting one to take advantage of the seasonal fluctuation in water
supply. A convenient form of these wheels includes draft tubes, by
which the wheel may be set several feet above the tailrace, and the
advantage of this additional fall still be preserved. In this case the
draft tube must be airtight so as to form suction, when filled with
escaping water, and should be proportioned to the size of the wheel.
Theoretically these draft tubes might be 34 feet long, but in practice
it has been found that they should not exceed 10 or 12 feet under
ordinary circumstances. They permit the wheel to be installed on the
main floor of the power station, with the escape below, instead of
being set just above the tailrace level itself, as is the case when
draft tubes are not used.

Reaction turbines when working under a variable load require water
governors (like impulse wheels) although where the supply of water is
large, and the proportion of power between water wheel and dynamo is
liberal--say two to one, or more--this necessity is greatly reduced.
Reaction wheels as a rule govern themselves better than impulse
wheels, due both to the fact that they use more water, and that they
operate in a small airtight case. The centrifugal ball governor is the
type usually used with reaction wheels as well as with impulse wheels.
This subject will be discussed more fully later.

_Installing a Power Plant_

In developing a power prospect, the dam itself is usually not the site
of the power plant.