iameter 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
Appendix gives for every degree of the thermometer and every half-inch
of the barometer, the proper quantity to be added or subtracted for the
reduction of the observed height to the standard temperature of the
mercury at 32 deg. Fahr.

After these the index correction should be applied. This is the amount
of difference between the particular instrument and the readings of the
Royal Society's flint-glass barometer when properly corrected, and is
generally known as the _zero_. It is impossible to pay too much
attention to the determination of this point. For this purpose, when
practicable, the instrument should be immediately compared with the
Royal Society's standard, and the difference of the readings of both
instruments, when corrected as above, carefully noted and preserved.
Where, however, this is impracticable, the comparison should be effected
by means either of some other standard previously so compared, or of an
intermediate portable barometer, the zero point of which has been _well
determined_. Suspend the portable barometer as near as convenient to the
ship's barometer, and after at least an hour's quiet exposure, take as
many readings of both instruments as may be necessary to reduce the
probable error of the mean of the differences below 0.001 inch. Under
these circumstances the mean difference of all the readings will be the
_relative_ zero or index error, whence, if that of the intermediate
barometer be known, that of the other may be found. As such comparisons
will always be made when the vessel is in port, sufficient time can be
allowed for making the requisite number of observations: hourly readings
would perhaps be best, and they would have the advantage of forming part
of the system when in operation, and might be accordingly used as suc