an thirty inches long; the measurement being taken
from the junction of the exhaustion branch with the fall-tube to the top
of the turned-up end.

[Illustration: Fig. 12]

_A Macleod Pump._--One form of this is illustrated by _b_, Fig. 12. It
has the advantage that the mercury reservoir may be allowed to become
empty without affecting the vacuum in the vessel being exhausted.

_"Spinning" Glass._--By the use of suitable appliances, it is quite
possible to draw out a continuous thread of glass, which is so thin as
to have almost the flexibility and apparent softness of woollen fibre; a
mass of such threads constitutes the "glass wool" of commerce.

The appliances necessary are:--a blowpipe capable of giving a
well-formed flame of about six or eight inches in length, a wheel of
from eighteen inches to three feet in diameter and having a flat rim of
about three inches wide, and a device for rotating the wheel at a speed
of about three hundred revolutions per minute.

A very satisfactory arrangement may be made from an old bicycle; the
back wheel having the tyre removed and a flat rim of tin fastened on in
its place. The chain drive should be retained, but one of the cranks
removed and a handle substituted for the remaining pedal. The whole
device is shown by Fig. 13.

[Illustration: Fig. 13]

The procedure in "spinning" glass is as follows:--First melt the end of
a glass rod and obtain a large mass of thoroughly softened glass, now
spin the wheel at such a speed that its own momentum will keep it
spinning for several seconds. Touch the end of the melted rod with
another piece of glass and, without withdrawing the original rod from
the blowpipe flame, draw out a thread of molten glass and twist it round
the spinning wheel. If this is done properly, the thread of glass will
grip on the flat rim, and by continuing to turn the wheel by hand it is
possible to draw out a continuous thread from the melted rod, which must
be advanced in the blowpipe flame as it is drawn on the wheel. If the
rod is not advanced sufficiently the thread will melt off, if it is
advanced too much, so as to heat the thick part and allow the glass to
become too cool at the point of drawing out, then the thread will become
too thick, but it is easy after a little practice to obtain the right
conditions. Practice is necessary also in order to find the right speed
for the wheel.

When sufficient glass has been "spun," the whole "hank" of thin threa