ANEROID BAROMETER. This instrument consists essen tially of a vacuum chamber, two opposite walls of which are thin metal membranes, which respond elastically to any variation of pressure on their outer faces, their movement being controlled by a spring and communicated to the indicating needle. An instru ment depending on a similar principle is the Bourdon pressure gauge (q.v.). The aneroid barometer is not so accurate as the mercury barometer, but it is often employed when the use of the latter would be inconvenient. (See BAROMETER. )