an hour, unless so constructed that the centrifugal force of the rim will
be counteracted, to a material extent, by the centripetal action of the
arms. Hooped wheels are very unsafe, unless the hoops are, by some process
or other, firmly attached to the arms. It is of no use to increase the
dimensions of the rim of a wheel with the view of giving increased strength
to counteract the centrifugal force, as every increase in the weight of the
rim will increase the centrifugal force in the same proportion.



CENTRES OF GRAVITY, GYRATION, AND OSCILLATION.

31. _Q._--What do you understand by the centre of gravity of a body?

_A._--That point within it, in which the whole of the weight may be
supposed to be concentrated, and which continually endeavors to gain the
lowest possible position. A body hung in the centre of gravity will remain
at rest in any position.

32. _Q._--What is meant by the centre of gyration?

_A._--The centre of gyration is that point in a revolving body in which the
whole momentum may be conceived to be concentrated, or in which the whole
effect of the momentum resides. If the ball of a governor were to be moved
in a straight line, the momentum might be said to be concentrated at the
centre of gravity of the ball; but inasmuch as, by its revolution round an
axis, the part of the ball furthest removed from the axis moves more
quickly than the part nearest to it, the momentum cannot be supposed to be
concentrated at the centre of gravity, but at a point further removed from
the central shaft, and that point is what is called the centre of gyration.