There are few persons who have not been puzzled, when witnessing the
exhibitions of conjurers and performers of feats of legerdemain, by the
_magic bottle_, out of the neck of which the exhibitor can pour any one
of quite a number of liquids at his will. It may interest the reader to
see an explanation of the means by which the apparently magical effect
is produced, especially as it involves an explanation of a certain
philosophical principle which it is very useful for all to understand.
The pressure of the atmosphere all around us is so great that no liquid
can issue against it from a close vessel, unless air is at the same time
admitted to balance the external pressure by an internal one of the same
amount. In the case of pouring water from a bottle the mouth of which is
tolerably large, the air passes in in large bubbles as the water comes
out, producing the gurgling sound always heard in such a case.
Where the orifice is too small to allow of the admission of these
bubbles of air, the liquid will only flow out as fast as the air is
allowed to enter in some other way, as shown in the engraving, where the
water will not issue from the lower end of the tube except when the
finger is raised from the upper end so as to admit the air.
There are various ingenious contrivances by means of which curious
effects are produced through the operation of this principle. One,
called the magic tunnel, is made double, with a space inclosed between
the walls. There is an orifice communicating with this chamber at the
top of the handle, which orifice is so situated that it can be opened or
closed at pleasure by the thumb of the person holding it without
attracting the attention of the spectator. Now if the body of the tunnel
is filled, or partly filled, with pure water, while the hidden chamber
contains a liquid deeply colored--with cochineal, for example--the
person holding it can cause pure water to flow from it by keeping the
orifice in the handle closed by his thumb, or colored water by simply
raising his thumb and allowing the liquid in the concealed chamber to
flow out and mingle with the clear water as it issues from the tube
below.
[Illustration: Fig. 1.]
The magic bottle acts on the same principle, though presenting it in
another form. The bottle is usually made of tin, though colored on the
outer surface to represent glass. Within, it contains several different
receptacles, as shown in Fig. 1, each communicating by a separate pipe
with the mouth of the bottle. Each of these receptacles is also provided
with another tube, by which air may be admitted so as to allow the
liquid contained in it to flow. These air tubes open by orifices in the
side of the bottle, as shown in Fig. 2, which are covered and concealed
by the thumb and the ends of the fingers of the operator, and may be
kept closed or may be slightly opened at pleasure. By this means any one
of five different liquids may be poured from the mouth of the bottle.
[Illustration: Fig. 2.]
Of course it requires some dexterity to manipulate such an apparatus
skillfully, in order to keep all the holes concealed from the
spectators, and to open the right one, just enough to admit the air, and
at the right time. The point of interest, however, for the general
reader in the whole subject is the philosophical principle which is
involved, namely, that the pressure of the atmosphere in every direction
all around us is such that no liquid can issue from any orifice against
the force of it acting _from without inward_, unless by the admission of
air or the providing by some other means of an equal force to act _from
within outward_ as a counterpoise.