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.
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