Fortin's Barometer PSAW An instrument to measure atmospheric pressure in laboratories. With metric and english system scales reading 25" to 32" and 64cm to 81cm. Vernier operating with rack and pinion, reads 0.1" and 0.1cm. Mercury level is adjusted by means of a knurled knob mounted at the bottom of the casing. When the knob is turned all the way into the casing, mercury fills the entire system and the tube accepts a very small volume to allow for thermal expansion. Borosilicate glass tube , fitted on wooden base supplied without mercury.