ing _E_ of the air-tube should be contracted to the size of a large
knitting needle. For producing a flame of large size, rich in air (Fig.
4, p. 9), the internal diameter of _E_ may be nearly half as great as
that of _A_ without disadvantage.

This blow-pipe may be fixed in position by the spike _F_, which will fit
into holes in a block of wood or a large cork. Several of these holes in
various positions should be made in the block, so that the position of
the blow-pipe may be varied easily. Two taps must be provided in
convenient positions near the edge of the table to enable the workman to
regulate the supplies of air and gas. These taps should be fixed to the
table and be connected with the gas and air supplies respectively on one
side, and with the blow-pipe on the other, by flexible tubes. If
blow-pipes of this kind be used, at least two of them should be
provided; one of small dimensions for working on small tubes and joints,
the other of larger size for operations on larger tubes. It will be
convenient to have both of them ready for use at all times, as it is
sometimes necessary to employ large and small flames on the same piece
of work in rapid succession. By having several air-tubes of different
sizes fitted to each blow-pipe, a greater variety of work may be done.

For the larger blow-pipe, the internal diameter of _A_ may be fifteen to
seventeen millimetres.

For the smaller instrument, eleven millimetres for the diameter of _A_
would be a useful size.

When a slightly greater outlay can be afforded it will be most
convenient to purchase the blow-pipe. They can be obtained of compact
form, supported on stands with universal joints giving great freedom of
movement, and with taps for regulating the supplies of gas and air, at
comparatively small cost.

As figures of various blow-pipes can be seen in the price-lists of most
dealers in apparatus, they are not given here. Their introduction would
be of but little service, for the construction of that which is adopted
can be readily ascertained by taking it to pieces. The simplest
blow-pipe usually used for glass-working is that of Herapath. This has
two taps to regulate the air and gas supplies respectively, and will
give a considerable variety of flames, which will be discussed
subsequently.

An excellent blow-pipe, made on the same principle as that shown in Fig.
1, but more substantially and with interchangeable jets, can be obtained
from Messrs. Muller