Pressure-gauge
A pressure-gauge for showing the amount of pressure in the wind-box and pipes of the cupola consists of a glass siphon tube, with equal legs, half filled with mercury; one end is cemented into a pipe which enters the wind-box, the other is open to the atmosphere. If a stop-cock is provided, it may be shut off at pleasure. The wind acting on the mercury in one leg of the gauge presses it down, and it rises correspondingly in the other leg. The difference between the two columns is the height of mercury which corresponds to the excess of the pressure in the wind-box above the pressure of the atmosphere. If eight ounces per inch be allowed for the length of this column, the effective pressure of the blast in ounces per square inch is obtained. Therefore all that is necessary for graduating the tube is to mark it off in one-eighth-inch divisions, and each division would represent one ounce of pressure. See BLAST-GAUGE.