the mercury must have left the cistern, and consequently must have
_lowered_ the surface in the cistern: in this case the altitude as
measured by the scale will be _too short--vice versâ_, if below. The
relation of the capacities of the tube and cistern should be
experimentally ascertained, and marked upon the instrument by the maker.
Suppose the capacity to be 1/50, marked thus on the instrument,
"_Capacity 1/50:_" this indicates that for every inch of variation of
the mercury in the tube, that in the cistern will vary contrariwise
1/50th of an inch. When the mercury in the tube is _above_ the neutral
point, the difference between it and the neutral point is to be reduced
in the proportion expressed by the "capacity" (in the case supposed,
divided by 50), and the quotient _added_ to the observed height; if
_below, subtracted_ from it. In barometers furnished with a fiducial
point for adjusting the lower level, this correction is superfluous, and
must not be applied.

The second correction required is for the capillary action of the tube,
the effect of which is always to depress the mercury in the tube by a
certain quantity inversely proportioned to the diameter of the tube.
This quantity should be experimentally determined during the
construction of the instrument, and its amount marked upon it by the
maker, and is always to be _added_ to the height of the mercurial
column, previously corrected as before. For the convenience of those who
may have barometers, the capillary action of which has not been
determined, a table of corrections for tubes of different diameters is
placed in the Appendix, Table I.

The next correction, and in some respects the most important of all, is
that due to the temperature of the mercury in the barometer tube at the
time of observation, and to the expansion of the scale. Table II. of the