But there is a very startling reflection connected with this
consideration. How strange it is that the vast 'substantial fabric' of
the earth should, after all, present itself as one grand source of
motion in terrestrial things! Gravitation, weight, the majestic
influence that holds the stable pyramid upon its base through centuries
of time, condescending to turn the restless wheels of man's machinery!
When the expansive burst of the vapour confined within the cylinder of
the condensing steam-engine thrusts upwards the piston-rod with its
mighty beams, it is simple weight--the weight of the superincumbent
transparent atmosphere--that crushes the metal back with antagonistic
force. When particles of water have been sublimated into the air by the
heating power of the solar rays, it is simple weight--the weight of
their own aqueous substance--that brings them down again, and that
causes their falling currents to turn the countless mill-wheels
implanted in the direction of their descent. When isolated tracts of the
atmosphere have been rendered rare and light under the concentrated
warmth of the sun, it is simple weight--the weight of colder and heavier
portions of the air--that makes winds rush into the spots where the
deficient downward pressure is, and that causes the sails of innumerable
windmills to whirl before the impulse of the breeze.

In the steam-engine we see the earth's gravitation and artificial heat
combining to effect sundry useful purposes, requiring enormous
expenditure of effort. In windmills and watermills we see the earth's
gravitation and natural or solar heat working together to perform like
service. In the pendulum, the earth's gravitation acting alone as an
enumerator of passing moments; for the momentum conferred by motion is
after all but a secondary result, an offspring of the earth's
attraction. In the steady oscillations of this little instrument no less