Source : Webster's Revised Unabridged Dictionary (1913)

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.