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.

[Illustration: FIG. 150.--Smoke-making apparatus.]

A ring of smoke is projected from the hole in the card if the rubber
diaphragm is pushed inwards. A slow, steady push makes a fat, lazy ring
come out; a smart tap a thinner one, moving much fa