tudy of machines we
omitted a factor which in practical cases cannot be ignored, namely,
friction. No surface can be made perfectly smooth, and when a barrel
rolls over an incline, or a rope passes over a pulley, or a cogwheel
turns its neighbor, there is rubbing and slipping and sliding. Motion
is thus hindered, and the effective value of the acting force is
lessened. In order to secure the desired result it is necessary to
apply a force in excess of that calculated. This extra force, which
must be supplied if friction is to be counteracted, is in reality
waste work.

If the force required by a machine is 150 pounds, while that
calculated as necessary is 100 pounds, the loss due to friction is 50
pounds, and the machine, instead of being thoroughly efficient, is
only two thirds efficient.

Machinists make every effort to eliminate from a machine the waste due
to friction, leveling and grinding to the most perfect smoothness and
adjustment every part of the machine. When the machine is in use,
friction may be further reduced by the use of lubricating oil.
Friction can never be totally eliminated, however, and machines of
even the finest construction lose by friction some of their
efficiency, while poorly constructed ones lose by friction as much as
one half of their efficiency.

170. Man's Strength not Sufficient for Machines. A machine, an inert
mass of metal and wood, cannot of itself do any work, but can only
distribute the energy which is brought to it. Fortunately it is not
necessary that this energy should be contributed by man alone, because
the store of energy possessed by him is very small in comparison with
the energy required to run locomotives, automobiles, sawmills, etc.
Perhaps the greatest value of machines lies in the fact that they
enable man to perform work by the use of energy other than his own.

[Illustration: FIG. 118.--Man's strength is not sufficient for heavy
work.]

Figure 118 shows one way in which a horse's energy can be utilized in
lifting heavy loads. Even the fleeting wind has been harnessed by man,
and, as in the windmill, made to work for him (Fig. 119). One sees
dotted over the country windmills large and small, and in Holland, the
country of windmills, the landowner who does not possess a windmill is
poor indeed.

For generations running water from rivers, streams, and falls has
served man by carrying his logs downstream, by turning the wheels of
his mill, etc.; and in our own