§ 152. Hydraulic machines may be broadly divided into two classes: (1)
_Motors_, in which water descending from a higher to a lower level, or
from a higher to a lower pressure, gives up energy which is available
for mechanical operations; (2) _Pumps_, in which the energy of a steam
engine or other motor is expended in raising water from a lower to a
higher level. A few machines such as the ram and jet pump combine the
functions of motor and pump. It may be noted that constructively pumps
are essentially reversed motors. The reciprocating pump is a reversed
pressure engine, and the centrifugal pump a reversed turbine. Hydraulic
machine tools are in principle motors combined with tools, and they now
form an important special class.
Water under pressure conveyed in pipes is a convenient and economical
means of transmitting energy and distributing it to many scattered
working points. Hence large and important hydraulic systems are adopted
in which at a central station water is pumped at high pressure into
distributing mains, which convey it to various points where it actuates
hydraulic motors operating cranes, lifts, dock gates, and in some cases
riveting and shearing machines. In this case the head driving the
hydraulic machinery is artificially created, and it is the convenience of
distributing power in an easily applied form to distant points which
makes the system advantageous. As there is some unavoidable loss in
creating an artificial head this system is most suitable for driving
machines which work intermittently (see POWER TRANSMISSION). The
development of electrical methods of transmitting and distributing energy
has led to the utilization of many natural waterfalls so situated as to
be useless without such a means of transferring the power to points where
it can be conveniently applied. In some cases, as at Niagara, the
hydraulic power can only be economically developed in very large units,
and it can be most conveniently subdivided and distributed by
transformation into electrical energy. Partly from the development of new
industries such as paper-making from wood pulp and electro-metallurgical
processes, which require large amounts of cheap power, partly from the
facility with which energy can now be transmitted to great distances
electrically, there has been a great increase in the utilization of
water-power in countries having natural waterfalls. According to the
twelfth census of the United States the total amount of water-power
reported as used in manufacturing establishments in that country was
1,130,431 h.p. in 1870; 1,263,343 h.p. in 1890; and 1,727,258 h.p. in
1900. The increase was 8.4% in the decade 1870-1880, 3.1% in 1880-1890,
and no less than 36.7% in 1890-1900. The increase is the more striking
because in this census the large amounts of hydraulic power which are
transmitted electrically are not included.