[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 faster. Absolutely still
air is needed for the best effects, as draughts make the rings lose shape
very quickly and move erratically. Given good conditions, a lot of fun can
be got out of the rings by shooting one through another which has expanded
somewhat, or by destroying one by striking it with another, or by
extinguishing a candle set up at a distance, and so on. The experimenter
should notice how a vortex ring rotates in itself while moving forward,
like a rubber ring being rolled along a stick.
A continuous supply of smoke can be provided by the apparatus shown in Fig.
150. The bulb of a scent spray is needed to force ammonia gas through a
box, made air-tight by a rubber band round the lid, in which is a pad
soaked with hydrochloric acid. The smoke formed in this box is expelled
through a pipe into the ring-making box.
Caution.
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