3. Column height

The height h of the column, which is the difference between the levels of the upper and lower meniscus, requires precise determination of these two levels.

Different methods are used to determine the level of the lower meniscus of mercury, depending on the type of barometer used.

— For Fortin-type barometers, the level of the lower meniscus is adjusted to a well-defined height, by bringing this meniscus flush with the end of an ivory tip attached to the case. When the barometer is correctly positioned (vertical tube), the zero point of the scale graduation on the ruler is, by construction, in the same horizontal plane as this tip, which precisely defines the point of application of the barometric pressure.

— For barometers with a U-shaped tube, the ruler is adjustable in height, and the zero point of the scale is...