There is shown to the audience a singular instrument composed of glass,
two bulbs connected by two tubes.
[Illustration: Fig. 106.]
Water, which may be tinctured with indigo for the better effect, flows
from the bulb A very slowly into the bulb B, whence it quickly and
plainly runs up by the tortuous tubing, so thin as to scarcely let a
hair pass through it back into the bulb A. The drops of ascending water
are separated by air bubbles, so that the current can be clearly
studied.
Now, though the laws of nature forbid that water can by any power in
itself lift itself up to the height of the reservoir originally holding
it, here seems a contradiction. For a time, friction and the resistance
of the air appears to be done away with. But this paradox is readily
accounted for after close watching.
It will be found that the descending liquid does not ascend from ball B
into the winding tube without part of it being left in that ball. It is
this filling the space which gradually forces the air upwards (since it
cannot go down through the column of water from A). The only cause of
the liquid rising beyond B is its being filled, and once it is full the
movement must stop.
The same appearances of drops of water divided by air bubbles is shown
in the windows of filter-dealers, who use great lengths of bent glass
piping for the purpose of display; but a small force-pump is the active
agent as regards their apparatus.