Inertia is defined as the tendency of a body to persevere in its state either of rest or motion. It is generally used in the sense of persisting in a state of rest. Among the many illustrations of this property of matter, is one which figured in the text-books of thirty or forty years ago, and which the boys of that time adapted to their amusement by constructing the apparatus here illustrated.
It consists of three parts: the board which forms the base, a post about six inches high, and a strip of stout whalebone, or dry, elastic wood.
The board should be as much as seven-eighths of an inch in thickness, and the elastic strip or spring should be firmly inserted in an inclined slit cut through the board. The places of the spring and post should be so adjusted to each other, that when the latter is secured solidly by a good-sized screw passing up through the board, the former will press with its upper end against the top of the post (as shown by the dotted line) with some degree of force.
The top of the post should be hollowed slightly, to retain the ball; and the appearance of the whole will be improved by a coat of shellac or paint.
Now place a card on the top of the post; and if it is sufficiently level, a marble or bullet may be induced to remain on it, directly over the column; if not, a large bean, a spool, or a coin, will prove more tractable. Draw back the spring with the thumb and finger, as in the illustration; let it go suddenly, and it will snap the card away, leaving the superimposed object resting quietly on the top of the column.
The same principle is sometimes illustrated by balancing a card on the finger, placing a coin on the card, and snapping away the card with the other hand, the coin remaining on the finger.
Another way is to pile up a small tower with “checkers” or “draughts.” By a quick blow with a ruler, one checker may be knocked from between the others, without overturning the tower.
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