BAROMETER

instrument for measuring atmospheric pre ure. Galileo’s observation that water ordinarily rises in a pump to a height of about 34 ft. induced Torr i cell i to prove experimentally that the atmosphere has weight. He demonstrated that mercury filling a tube 3 ft. long sealed at one end, when inverted in a basin containing the same fluid sank to a level of about 30 in. above that of the mercury in the basin, a vacuum being left at the top of the tube. Subsequently, Pascal showed that the level of the mercury varied in different altitudes and in different weather, thus making Torr i cell i’s appliance, provided with a scale, known as a weather gla and an instrument for determining altitudes. In the so-called cistern barometer the mercury level in the basin naturally varies with the level in the tube, render- | ing the exact observation of different pre ures difficult owing to this ‘error of capacity.’ This is obviated in the Fortin barometer by a screw, by means of which the mercury in the cistern can be adjusted to a standard level indicated by anivory point. See ANEROID.