mall bubble of air to escape up the tube, also any irregularity or
dirt in the tube at this point makes it more easy for the bubbles of
air to escape to the surface of the mercury.

Any pump in which the supply of mercury to the fall tube can be
regulated nicely will pump well until the lowest available pressures
are being attained; a good pump will then continue to hold the air
bubbles, while a bad one will allow them to slip back [Footnote: For
special methods of avoiding this difficulty see Mr. Ram's book.] ...

Though three fall tubes are recommended, it must not be supposed that
the pump will produce a Crooke's vacuum three times more rapidly than
one fall tube. Until the mercury commences to hammer in the pump the
three tubes will pump approximately three times faster than one tube,
but as soon as the major portion of the air collected begins to come
from the layer condensed on the glass surface of the tube to be
exhausted and from the electrodes, the rate at which exhaustion will
go on no longer depends entirely on the pump.

In order that bubbles of air may not slip back up the fall tube it is
generally desirable to allow the mercury to fall pretty briskly, and
in this case the capacity of the pump to take air is generally far in
excess of the air supply. One advantage of having more than one fall
tube is that it often happens that a fall tube gets soiled during the
process of exhaustion and no longer works up to its best performance.
Out of three fall tubes, however, one is pretty sure to be working
well, and as soon as the mercury begins to hammer in the tubes the
supply may be shut off from the two falls which are working least
satisfactorily.

Thus we are enabled to pump rapidly till a high degree of exhaustion
is attained, having practically three pumps instead of one, whereas
when the final stages are reached, and three pumps are only a drawback
in that they increase the mercury flow, the apparatus is capable of
instant modification to meet the new conditions.

The thistle funnels at the head of the fall tubes are made simply by
blowing bulbs and then blowing the heads of the bulbs into wider ones,
and finally blowing the heads of the wider bulbs off by vigorous
blowing. The stoppers are ground in on the lathe before the tubes are
attached to the fall tubes. The stoppers require to be at least half
an inch long where they fit the necks, and must be really well ground
in. The stoppers must first