barometer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument for measuring atmospheric pressure, used especially in weather forecasting.
2. n. Something that registers or responds to fluctuations; an indicator: Opinion polls serve as a barometer of the public mood.

Century Dictionary and Cyclopedia

1. n. An instrument for measuring the weight or pressure of the atmosphere, invented by Evangelista Torricelli, an Italian mathematician and physicist, in 1643. The simplest form of this instrument is a glass tube over 30 inches long, sealed at one end, and then filled with mercury. When the tube is inverted, with the open end dipping into a cup or cistern of mercury, the column sinks, leaving a vacuum at the top, till the pressure of the atmosphere on each unit of surface of the mercury in the cistern equals the weight of the column in the tube over each unit of surface of the horizontal section at the level of the mercury outside, when the pressure of the column of mercury just balances that of the atmosphere. The rise and fall can be measured on a graduated scale. Barometers of this form are called cistern barometers. They are the commonest of rough mercurial barometers. For scientific purposes, the most frequently used is Fortin's barometer, in which the cistern is adjustable, the zero of the scale coinciding with the extremity of an ivory pointer (a in second figure) which projects downward from the top of the cistern-box. The bottom of the cistern is made of leather, and by a screw working against a wooden button the mercury can be raised or lowered until its surface just touches the point of the index; this operation must be performed before each observation. The siphon barometer consists of a bent tube, generally of uniform bore, having two unequal legs. The longer leg, which must be more than 30 inches long, is closed, while the shorter leg is open; the difference of the levels in the two legs represents the pressure of the atmosphere. The wheel barometer usually consists of a siphon barometer having a float resting on the surface of the mercury in the open branch, and a thread attached to the float passing over a pulley, and having a weight at its extremity as a counterpoise to the float. As the mercury rises and falls the thread turns the pulley which moves the index of the dial. The barometer is used in many physical and chemical determinations, but its most ordinary applications are to the prediction of changes in the weather, and to the determination of the elevation of stations above the sea-level.

Wiktionary

1. n. An instrument for measuring atmospheric pressure.
2. n. Anything used as a gauge or indicator.

GNU Webster's 1913

1. n. An instrument for determining the weight or pressure of the atmosphere, and hence for judging of the probable changes of weather, or for ascertaining the height of any ascent.

WordNet 3.0

1. n. an instrument that measures atmospheric pressure