the anchor has the same principle. The kitchen coffee
grinder and the meat chopper are other familiar illustrations.

Cogwheels are modifications of the wheel and axle. Teeth cut in _A_
fit into similar teeth cut in _B_, and hence rotation of _A_ causes
rotation of _B_. Several revolutions of the smaller wheel, however,
are necessary in order to turn the larger wheel through one complete
revolution; if the radius of _A_ is one half that of _B_, two
revolutions of _A_ will correspond to one of _B_; if the radius of _A_
is one third that of _B_, three revolutions of _A_ will correspond to
one of _B_.

[Illustration: FIG. 113.--Cogwheels.]

Experiment demonstrates that a weight _W_ attached to a cogwheel of
radius 3 can be raised by a force _P_, equal to one third of _W_
applied to a cogwheel of radius 1. There is thus a great increase in
force. But the speed with which _W_ is raised is only one third the
speed with which the small wheel rotates, or increase in power has
been at the decrease of speed.

This is a very common method for raising heavy weights by small force.

Cogwheels can be made to give speed at the decrease of force. A heavy
weight _W_ attached to _B_ will in its slow fall cause rapid rotation
of _A_, and hence rapid rise of _P_. It is true that _P_, the load
raised, will be less than _W_, the force exerted, but if speed is our
aim, this machine serves our purpose admirably.

An extremely important form of wheel and axle is that in which the two
wheels are connected by belts as in Figure 114. Rotation of _W_
induces rotation of _w_, and a small force at _W_ is able to overcome
a large force at _w_. An advantage of the belt connection is that
power at one place can be transmitted over a considerable distance and
utilized in another place.

[Illustration: FIG. 114.--By means of a belt, motion can be
transferred from place to place.]

166. Compound Machines. Out of the few simple machines mentioned in
the preceding Sections has developed the complex machinery of to-day.
By a combination of screw and lever, for example, we obtain the
advantage due to each device, and some compound machines have been
made which combine all the various kinds of simple machines, and in
this way multiply their mechanical advantage many fold.

A relatively simple complex machine called the crane (Fig. 116) maybe
seen almost any day on the street, or wherever heavy weights are being
lifted. It is clear that a force applie