PIEZOMETER (Gr. piezo, I press; metron, it measure), an instrument for measuring the compressibility of fluids. Oersted's (q.v.) instrument, the first by which the com pressibility of water was satisfactorily determined, consisted of a cylindrical glass jar. into the neck of which a narrower cylindrical tube of glass, open at both ends, was firmly fixed. In this tube 'worked an air-tight piston by means of a screw. In the interior of the jar was placed a bottle, whose neck was drawn out into a long capillary graduated tube, and alongside this bottle was suspended a cylindrical tube, closed at the top, but open at the bottom. When the compressibility of any liquid was to be determined, the instrument was adjusted in the following manner: the bottle inside was tilled almost to the top of the capillary tube with the fluid, and being replaced inside the jar, the latter was completely filled with water up to the piston in the neck. The liquid in the submerged bottle, then under, pressure of the water above it, fell slightly in the capillary tube, being kept from contact with the water by an air-bubble, the motion of which up or down, according as the pressure was less or greater, served as an index for reading off the graduation. The suspended tube alongside being at first only tilled with
air, the water rose in it to some extent, and by graduations on the tube it was made to indicate the pressure in atmospheresor parts of atmospheres. Pressure was now applied to the water in the jar by screwing down the piston; the compressed water communicated the pressure to the liquid in the bottle and to the air in the suspended tube; the descent of the air-bubble in the former indicating the amount of diminution in bulk the liquid had undergone (the capillary tube being graduated in inches and parts of inches, and each inch of tube being known to contain a certain fraction of the contents of the bot tle), while the ascent of the water iu the suspended tube showed the amount of pressure which had been applied.
PIG. See Hoo.