towards their proper places. All these seem to be natural motions,
dependent on the inherent faculties, or tendencies, of bodies
themselves. But there are other motions which are artificial or
violent, as when a stone is thrown from the hand, or is knocked by
another stone in motion. In such cases as these, for example, when a
stone is cast from the hand, the distance travelled by the stone
appears to depend partly on its weight and partly upon the exertion
of the thrower. So that, the weight of the stone remaining the same,
it looks as if the motive power communicated to it were measured by
the distance to which the stone travels--as if, in other words, the
power needed to send it a hundred yards was twice as great as that
needed to send it fifty yards. These, apparently obvious, conclusions
from the everyday appearances of rest and motion fairly represent the
state of opinion upon the subject which prevailed among the ancient
Greeks, and remained dominant until the age of Galileo. The
publication of the 'Principia' of Newton, in 1686-7, marks the epoch
at which the progress of mechanical physics had effected a complete
revolution of thought on these subjects. By this time, it had been
made clear that the old generalisations were either incomplete or
totally erroneous; that a body, once set in motion, will continue to
move in a straight line for any conceivable time or distance, unless
it is interfered with; that any change of motion is proportional to
the 'force' which causes it, and takes place in the direction in which
that 'force' is exerted; and that, when a body in motion acts as a
cause of motion on another, the latter gains as much as the former
loses, and _vice versâ_. It is to be noted, however, that while, in
contradistinction to the ancient idea of the inherent tendency to
motion of bodies, the absence of any such spontaneous power of motion
was accepted as a physical axiom by the moderns, the old conception
virtually maintained itself is a new shape. For, in spite of Newton's