Syphons are bent tubes for drawing off liquids from cisterns, butts,
&c., where there is no tap, and where it would be inconvenient to
make any second opening. Fig. 1 gives the outline of the most usual
form of syphon; these are only used for liquids that may be drawn
into the mouth without injury, such as spirits from casks. The mode
of using the syphon is this--the bottom of the longest leg _a_ is
stopped with the palm of the hand, the tap is then turned on and
the mouth applied to the small tube _c_, the air is then drawn out
by sucking; the liquid rises and fills both legs of the instrument,
the tap is turned off, and the syphon is full. Now as the leg _a_
is longer than the leg _b_, the fluid in it weighs more than that
in _b_, and sinking down draws the fluid in _b_ up, and so on till
all is drawn from the cask. Syphons are generally made of copper,
but gutta-percha would answer exceedingly well. Fig. 2 represents a
contrivance for drawing off acids, &c., which would injure the mouth;
the ball prevents the acid rising into it, as the mouth is removed
directly it begins to fill, which as the instrument is of glass, can
easily be seen.