ted 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 versa_. 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
well-known warning against the 'absurdity' of supposing that one body
can act on another at a distance through a vacuum, the ultimate
particles of matter were generally assumed to be the seats of
perennial causes of motion termed 'attractive and repulsive forces,'
in virtue of which, any two such particles, without any external
impression of motion, or intermediate material agent, were supposed to
tend to approach or remove from one another; and this view of the
duality of the causes of motion is very widely held at the present
day.

Another important result of investigation, attained in the seventeenth
century, was the proof and quantitative estimation of physical
inertia. In the old philosophy, a curious conjunction of ethical and
physical prejudices had led to the notion that there was something
ethically bad and physically obstructive about matter. Aristotle
attributes all irregularities and apparent dysteleologies in nature to
the disobedience, or sluggish yielding, of matter to the shapin