The Collaborative International Dictionary of English v.0.48:

Sympiesometer \Sym`pi*e*som"e*ter\, n. [Gr. ? compression (fr. ?
   to press together; sy`n with + ? to press, squeeze) +
   -meter.]
   A sensitive kind of barometer, in which the pressure of the
   atmosphere, acting upon a liquid, as oil, in the lower
   portion of the instrument, compresses an elastic gas in the
   upper part.
   [1913 Webster]

   Note: The column of oil of a lower part BC of a glass tube
         compresses hydrogen gas in the upper part AB, and is
         thus measured on the scale pq by the position of a
         surface of the oil in the tube. The scale pq is
         adjustable, and its index must be set to the division
         on the scale rs corresponding to the temperature
         indicated by the termometer t, in order to correct for
         the effects of temperature on the gas. It is sensitive,
         and convenient for use at sea, but inferior in accuracy
         to the mercurial barometer.
         [1913 Webster]