Whenever, then, a heavy body suspended by a flexible string is drawn to one side, and dropped from the hand, a vibrating pendulum is made, because weight and acquired impulse influence it alternately with a sort of see-saw action, the power of the one diminishing as the power of the other augments.  Weight pulls down—­confers velocity and impulse during the pulling—­and then velocity carries up.  As velocity carries up, weight diminishes its impulse, and at last arrests it, and then begins to pull down again.  In the middle of the vibration velocity is at its greatest, and weight at its least, as regards their influence on the motion.  At the extremes of the vibration velocity is at its least, and weight at its greatest.  Now here it is the earth’s attraction clearly that confers the impulse of the downward movement, just as much as it is the earth’s attraction that causes the downward movement of running water.  Therefore the power which makes the pendulum swing is the same with the power which grinds the corn in the water-mill—­the attraction of the earth’s vast mass for the mass of a smaller body placed near to its surface under certain peculiar conditions of position.

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