blowpipes. In order to provide a final adjustment for the flame, a
perforated plate having seven holes which correspond in size and
position to the outer tubes is arranged to slide on parallel guides in
front of these outer tubes.

[Illustration: Fig. 2]

The next piece of apparatus for consideration is the bellows, of which
there are three or more types on the market, although all consist of two
essential parts, the blower or bellows proper and the wind chamber or
reservoir. Two patterns are shown in Fig. 2; _a_, is the form which is
commonly used by jewellers and metal workers to supply the air blast
necessary for heating small furnaces. Such a bellows may be obtained at
almost any jewellers' supply dealer in Clerkenwell, but it not
infrequently happens that the spring in the wind chamber is too strong
for glass-blowing, and hence the air supply tends to vary in pressure.
This can be improved by fitting a weaker spring, but an easier way and
one that usually gives fairly satisfactory results, is to place an
ordinary screw-clip on the rubber tube leading from the bellows to the
blowpipe, and to tighten this until an even blast is obtained.

Another form of bellows, made by Messrs. Fletcher and Co., and common in
most laboratories, is shown by _b_; the wind chamber consists of a disc
of india-rubber clamped under a circular frame or tied on to a circular
rim. This form is shown by Fig. 2, _b_.

The third form, and one which my own experience has caused me to prefer
to any other, is cylindrical, and stands inside the pedestal of the
blowpipe-table. A blowpipe-table of this description is made by Enfer of
Paris.

There is no need, however, to purchase an expensive table for laboratory
use. All the work described in this book can quite well be done with a
simple foot bellows and a quick-change blowpipe. Nearly all of it can be
done with a single jet blowpipe, such as that described first, or even
with the still simpler apparatus mentioned on page 84, but I do not
advise the beginner to practise with quite so simple a form at first,
and for that reason have postponed a description of it until the last
chapter.

Glass-blowers' tools and appliances are many and various, quite a number
of them are better rejected than used, but there are a few essentials.
These are,--file, glass-knife, small turn-pin, large turn-pin, carbon
cones, carbon plate, rubber tube of small diameter, various sizes of
corks, and an asbestos heat