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, and will continue to swing in opposite directions—­that is, while A swings north or east B will be swinging south or west, and vice versa.

3.  Start them both in the same direction, but one from a higher point than the other.  There will be the same transference of energy as in (2), but neither will come to rest between alternations, and they will always swing in the same direction.

Unequal Lengths.—­If for one of the original pendulums we substitute one a couple of inches longer than the other, but of the same weight, the same set of three experiments will provide six variations among them, as in each case either the longer or the shorter may be started first or given the longer initial swing, as the case may be.  The results are interesting throughout, and should be noted.

Three or more Pendulums.—­If the number of pendulums be increased to three or more, the length of all being the same, a fresh field for observation is opened.  With an increase of number a decrease in the individual weighting is advisable, to prevent an undue sagging of the string.

In conclusion, we may remark that a strong chain stretched between two trees and a suitable supply of rope will enable the reader and his friends to carry out all the experiments on a life-size scale.

A Smoke-ring Apparatus.—­Get a large tin of the self-opening kind and cut a hole 2 inches across in the bottom.  Then make a neat circular hole 1-1/4 inches in diameter in the centre of a paper disc somewhat smaller than the bottom of the tin, to which it is pasted firmly on the outside.  The other end—­from which the lid is removed—­must be covered with a piece of sheet rubber stretched fairly tight and secured to the tin by string passed over it behind the rim.  An old cycle or motor car air tube, according to the size of the tin, will furnish the rubber needed; but new material, will cost only a few pence (Fig. 149).

[Illustration:  Fig. 149.—­Smoke-ring apparatus.]

A dense smoke is produced by putting in the tin two small rolls of blotting paper, one soaked in hydrochloric acid, the other in strong ammonia.  The rolls should not touch.  To reduce corrosion of the tin by the acid, the inside should be lined with thin card.