Placing the instrument under the receiver and forming a partial vacuum, the index will rise into the enlargement. Then, admitting the air by degrees and very slowly, we may note the correspondence between the heights of the ordinary mercury manometer and the point which will be reached by the lower face of the index of the instrument. This will answer to form a comparative table of the pressures and the numbers of the scale. The pressures would be represented by their relations to the observed pressure at the moment of the passage of the index over zero, for any other fixed number of the scale.
Thus, for example, suppose that we observed on the manometer 400 or n millimetres of mercury when the index is on o, then n' when the index is on 1, n" when on 2, and so on. This will give the ratios which must be inscribed in the table. Then n could be varied at pleasure, and the table could still be used.
In fact, according to the law of Mariotte, volumes preserving the same ratios, pressures should also preserve the same ratios to each other.
Let p be the pressure when the index is on o, v the volume of air at the same moment, p' and v' the same pressures and volume at the moment
