huge dams are built which hold back the water
and keep it at a higher level than the wheel; from the dam the water
is conveyed in pipes (flumes) to the paddle wheel which it turns.
Cogwheels or belts connect the paddle wheel with the factory
machinery, so that motion of the paddle wheel insures the running of
the machinery.

[Illustration: FIG. 121.--The Pelton water wheel.]

One of the most efficient forms of water wheels is that shown in
Figure 121, and called the Pelton wheel. Water issues in a narrow jet
similar to that of the ordinary garden hose and strikes with great
force against the lower part of the wheel, thereby causing rotation of
the wheel. Belts transfer this motion to the machinery of factory or
mill.

173. Turbines. The most efficient form of water motor is the
turbine, a strong metal wheel shaped somewhat like a pin wheel,
inclosed in a heavy metal case.

[Illustration: FIG. 122--A turbine at Niagara Falls.]

Water is conveyed from a reservoir or dam through a pipe (penstock) to
the turbine case, in which is placed the heavy metal turbine wheel
(Fig. 122). The force of the water causes rotation of the turbine and
of the shaft which is rigidly fastened to it. The water which flows
into the turbine case causes rotation of the wheel, escapes from the
case through openings, and flows into the tail water.

The power which a turbine can furnish depends upon the quantity of
water and the height of the fall, and also upon the turbine wheel
itself. One of the largest turbines known has a horse power of about
20,000; that is, it is equivalent, approximately, to 20,000 horses.

174. How much is a Stream Worth? The work which a stream can perform
may be easily calculated. Suppose, for example, that 50,000 pounds of
water fall over a 22-foot dam every second; the power of such a stream
would be 1,100,000 foot pounds per second or 2000 H.P. Naturally, a
part of this power would be lost to use by friction within the
machinery and by leakage, so that the power of a turbine run by a 2000
H.P. stream would be less than that value.

Of course, the horse power to be obtained from a stream determines the
size of the paddle wheel or turbine which can be run by it. It would
be possible to construct a turbine so large that the stream would not
suffice to turn the wheel; for this reason, the power of a stream is
carefully determined before machine construction is begun, and the
size of the machinery depends upon the