A device, consisting of a pipe or tube bent so as to form
   two branches or legs of unequal length, by which a liquid can be
   transferred to a lower level, as from one vessel to another, over an
   intermediate elevation, by the action of the pressure of the atmosphere
   in forcing the liquid up the shorter branch of the pipe immersed in it,
   while the continued excess of weight of the liquid in the longer branch
   (when once filled) causes a continuous flow. The flow takes place only
   when the discharging extremity of the pipe ia lower than the higher
   liquid surface, and when no part of the pipe is higher above the
   surface than the same liquid will rise by atmospheric pressure; that
   is, about 33 feet for water, and 30 inches for mercury, near the sea
   level.