A Pneumatic Puzzle.—­For the very simple apparatus illustrated by Fig. 147 one needs only half a cotton reel, three pins, and a piece of glass or metal tubing which fits the hole in the reel.  Adjust a halfpenny centrally over the hole and stick the pins into the reel at three equidistant points, so that they do not quite touch the coin, and with their ends sloping slightly outwards to allow the halfpenny to fall away.

[Illustration:  Fig. 147.—­Apparatus for illustrating an apparent scientific paradox.]

Press the coin against the reel and blow hard through the tube.  One would expect the coin to fall; but, on the contrary, the harder you blow the tighter will it stick, even if the reel be pointed downwards.  Only when you stop blowing will it fall to the floor.

This is a very interesting experiment, and will mystify onlookers who do not understand the reason for the apparent paradox, which is this.  The air blown through the reel strikes a very limited part of the nearer side of the halfpenny.  In order to escape, it has to make a right-angle turn and pass between coin and reel, and, while travelling in this direction, loses most of its repulsive force.  The result is that the total pressure on the underside of the coin, plus the effect of gravity, is exactly balanced by the atmospheric pressure on the outside, and the coin remains at that distance from the reel which gives equilibrium of forces.  When one stops blowing, 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.