The word “aneroid” means “without fluid,” and so aneroid barometers do not need mercury in order to work. French inventor Lucien Vidie (1805–1866) built upon a concept first proposed by German mathematician Gottfried Leibniz (1646–1716) in which a metallic capsule surrounded by a vacuum could be used to measure air pressure. Using very thin pieces of metal, Vidie managed to connect such a capsule to highly sensitive dials displayed behind glass within an encasement. This was highly detailed work on the level of the finest clock craftsmanship. Aneroid barometers were very difficult to make in Vidie’s time, but high-tech instruments are produced today using such devices as electron beams welding copper beryllium alloys. Because aneroid barometers are made of metal, they are also sensitive to changes in temperature and altitude. Bimetallic strips can be used to compensate for temperature, but altitude poses more of a problem. For this reason, aneroid barometers work best at elevations below 3,000 feet (about 915 meters), but they can be calibrated for higher altitudes if needed.