ed under heads of less than 20 feet.
A wheel of this type, six feet in diameter, would develop six
horsepower, with 188 cubic feet of water a minute and 20-foot head.
The great majority of impulse wheels are used under heads of 100 feet
and over. In this country the greatest head in use is slightly over
2,100 feet, although in Switzerland there is one plant utilizing a
head of over 5,000 feet.

[Illustration: Runner of Pelton wheel, showing peculiar shape of the
buckets]

[Illustration: The Fitz overshoot wheel

Efficient Modern Adaptations of the Archaic Undershot and Overshot
Water Wheels]

The old-fashioned impulse wheels were inefficient because of the fact
that their buckets were not constructed scientifically, and much of
the force of the water was lost at the moment of impact. The impulse
wheel of to-day, however, has buckets which so completely absorb the
momentum of water issuing from a nozzle, that the water falls into the
tailrace with practically no velocity. When it is remembered that the
nozzle pressure under a 2,250-foot head is nearly 1,000 pounds to the
square inch, and that water issues from this nozzle with a velocity of
23,000 feet a minute, the scientific precision of this type of bucket
can be appreciated.

A typical bucket for such a wheel is shaped like an open clam shell,
the central line which cuts the stream of water into halves being
ground to a sharp edge. The curves which absorb the momentum of the
water are figured mathematically and in practice become polished like
mirrors. So great is the eroding action of water, under great
heads--especially when it contains sand or silt--that it is
occasionally necessary to replace these buckets. For this reason the
larger wheels consist merely of a spider of iron or steel, with each
bucket bolted separately to its circumference, so that it can be
removed and replaced easily. Usually only one nozzle is provided; but
in order to use this wheel under low heads--down to 10 feet--a number
of nozzles are used, sometimes five, where the water supply is
plentiful.

The wheel is keyed to a horizontal shaft running in babbited bearings,
and this same shaft is used for driving the generator, either by
direct connection, or by means of pulleys and a belt. The wheel may be
mounted on a home-made timber base, or on an iron frame. It takes up
very little room, especially when it is so set that the nozzle can be
mounted under the flooring. The wheel itself i