Recognizing the importance of measuring very accurately the barometric
pressure, or at least its fluctuations, we have installed an accurate
barometer of the Fortin type, made by Henry J. Green. This is attached
to the inner wall of the calorimeter laboratory, and since the
calorimeter laboratory is held at a constant temperature, temperature
corrections are unnecessary, for we have here to deal not so much with
the accurate measurement of the actual pressure as with the accurate
measurement of differences in pressure. For convenience in reading, the
ivory needle at the base of the instrument and the meniscus are well
illuminated with electric lamps behind a white screen, and a small lamp
illuminates the vernier. The barometer can be read to 0.05 millimeter.