[Illustration: FIG. 1.]
The experiment shown in Fig. 1 requires no other apparatus than a
decanter and a strong piece of straw. The straw is bent before being
passed into the bottle of water, so that when it is lifted the centre of
gravity is displaced, and brought directly under the point of
suspension. The illustration shows the method of lifting the decanter of
water by the straw very plainly. It is well to have at hand several
pieces of straw, perfectly intact, and free from cracks, in case the
experiment does not succeed with the first attempt.
[Illustration: FIG. 2.]
The experiment shown in Fig. 2 is apparently very difficult, but it will
be found easy enough in practice if the hand be steady. Take a key, and
by means of a crooked nail, or "holdfast," attach it to a bar of wood by
a string tied tightly round the bar, as in the picture. To the other
extremity of the bar attach a weight, and then drive a large-headed nail
into the table. It will be found that the key will balance, and even
move upon the head of the nail, without falling. The weight is under the
table, and the centre of gravity is exactly beneath the point of
suspension.
Figs, 3 and 4 are examples of the force of inertia; that is, the
tendency of a thing that is at rest to remain in that state.
[Illustration: FIG. 3.]
To perform the experiment in Fig. 3 a needle is fixed at each end of a
broomstick, and these needles are made to rest on two glasses, placed on
chairs; the needles alone must be in contact with the glasses. If the
broomstick is then struck violently with another stout stick, the former
will be broken, but the glasses will remain intact. The experiment
answers all the better the more energetic the action. It is explained by
the resistance of inertia in the broomstick. The shock suddenly given,
the impulse has not time to pass on from the particles directly affected
to the adjacent particles; the former separate before the movement can
be transmitted to the glasses serving as supports.
[Illustration: FIG. 4.]
It is possible, for the same reason, to extract from a pile of money a
piece placed in the middle of the pile without overturning the others.
It suffices to move them forcibly and quickly with a flat wooden ruler.
The experiment succeeds very well also if performed with draughtsmen
piled up on the draught-board, Fig. 4.