Such a body is found in the pendulum, and a very intricate
mutual action exists between the pendulum and the scape wheel.
The function of the pendulum is to regulate and determine the
periods and amount of onward motion in the scape wheel, whereas
the office of the wheel is to impart such an impulse to the pendu-
lum at each period of this onward movement as may serve to
maintain its swing unimpaired, and may cause it to move with the
same mathematical precision which would characterise the vibra-'
tions of a body swinging in vacuo, and uninfluenced by any dis-
turbing causes.
It is remarkable that in the case of the ideal pendulum, where
there is no artificial resistance and no friction, the movement is in
theory perpetual In the case of the rigid pendulum the friction
at the point of suspension and the resistance of the air would gra-
dually diminish the arc of swing, and the movement would slowly '
subside and die away, although it might be many hours before it
became quite imperceptible. The mechanism of a clock must
therefore so act upon the pendulum as to maintain its swing un-
impaired by these resistances, and it should be borne in mind that
the swing of the pendulum is identical with that of the anchor in
the last-mentioned escapement, whence it follows that any impulse
or check given to the anchor is felt at once as an impulse or check
upon the pendulum.
