Barometers of the aneroid (without liquid) type are commonly used to forecast the weather in the short term, the idea being that low or falling pressure generally indicates unsettled or worsening weather while high or increasing pressure is generally associated with good or improving weather.
The essential part of an aneroid barometer is a flat cylindrical metal box or capsule, corrugated for strength and hermetically sealed after having been partially exhausted of air. An increase in atmospheric pressure causes the box to cave in slightly, while a decrease causes it to expand. The movements of the box re magnified by a system of levers and transmitted to a fine chain wrapped around the axis of a pointer. The chain is kept fixed by means of a hairspring attached to the spindle while the pointer moves over a suitably calibrated scale.
The same construction is used to make altimeters for aircraft - the instrument in calibrated in terms of altitude instead of weather conditions. The atmospheric pressure falls by 100mm of mercury per 120 m increase in altitude in the lower atmosphere.