ing, the air pressure on both sides is the same, and gravity makes the
coin fall away.

The function of the pins is merely to keep the halfpenny centred on the
hole. If steam is used instead of human breath, a considerable weight may
be hung from the disc without dislodging it.

The Magic Swingers.--The easily made toy illustrated next is much more
interesting than would appear from the mere picture, as it demonstrates a
very striking physical phenomenon, the transference of energy. If two
pendulums are hung close together from a flexible support and swung, their
movements influence one another in a somewhat remarkable way--the swing of
the one increasing as that of the other dies down, until a certain point is
reached, after which the process is reversed, and the "dying" or "dead"
pendulum commences to come to life again at the expense of the other. This
alternation is repeated over and over again, until all the energy of both
pendulums is exhausted.

[Illustration: FIG. 148.-Magic pendulums.]

To make the experiment more attractive, we substitute for the simplest
possible pendulums--weights at the end of strings--small swings, each
containing a figure sitting or standing on a seat, to the underside of
which is attached a quarter of a pound of lead. To prevent the swings
twisting, they are best made of strong wire bent as shown in Fig. 148, care
being taken that the sides are of equal length, so that both hooks may
press equally on the strings. Eighteen inches is a good length. The longer
the swing, and the heavier the weight, the longer will the experiment last.

The swings are hung, six inches apart, from a stout string stretched
tightly between two well-weighted chairs or between two fixed points. The
string should be at least 4 feet long.

With two equally long and equally weighted pendulums, the three following
experiments may be carried out:--

1. Let one, A, start from rest. The other, B will gradually die, and A
swing to and fro more and more violently, till B at last comes to a dead
stop. Then A will die and B in turn get up speed. The energy originally
imparted to B is thus transferred through the string from one pendulum to
the other an indefinite number of times, with a slight loss at every
alternation, until it is finally exhausted by friction.

2. Swing them in opposite directions, but start A from a higher point than
B. They will each alternately lose and gain motion, but will never come to
rest,