This enlargement shows the left half of the apparatus with a break in the middle.
The volume of gas between the mercury column and stopcock M in the right tube, which is about 4 feet long, would be measured by the calibrations on the tube. Note the Roman numerals from I near the top to VIII near the bottom which denote cubic inches. The finest graduations measure 1/100 cubic inch.
By adding mercury to the left tube from the funnel R, or draining it from S, the height of mercury and thus the pressure in the two columns could be equalized. It is interesting that the left and right tubes are of different diameter, meaning that the contribution of the mercury meniscus to the pressure would be different in the two tubes, a fact that would have to be accounted for in the calculations.
