ster. 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.--When dealing with hydrochloric acid, take great care not to get
it on your skin or clothes, as it is a very strong corrosive.

XXVII. A RAIN-GAUGE.

The systematic measurement of rainfall is one of those pursuits which prove
more interesting in the doing than in the prospect. It enables us to
compare one season or one year with another; tells us what the weather has
been while we slept; affords a little mild excitement when thunderstorms
are about; and compensates to a limited extent for the disadvantages of a
wet day.

The general practice is to examine the gauge daily (say at 10 a.m.); to
measure the water, if any, collected during the previous twenty-four hours;
and to enter the record at once. Gauges are made which record automatically
the rainfall on a chart or dial, but these are necessarily much more
expensive than those which merely catch the water for measurement.

This last class, to which our attention will be confined chiefly, all
include two principal parts--a metal receiver and a graduated glass
measure, of much smaller diameter than the receiver, so that the divisions
representing hundredths of an inch may be far enough apart to be
distinguishable. It is evident that the smaller the area of the measure is,
relatively to that of the receiver, the more widely spaced will the
graduation marks of the measure be, and the more exact the readings
obtained.

[Illustration: FIG. 151.--Standard rain-gauge.]

The gauge most commonly used is that shown in Fig. 151. It consists of an
upper cylindrical part, usually 5 or 8 inches in diameter, at the inside of
the rim, with i