verified on every opportunity which presents itself; and in every
instance, previous to sailing, it should be re-compared with the
standard on shore by the intervention of a portable barometer, and no
opportunity should be lost of comparing it on the voyage by means of
such an intermediate instrument with the standard barometers at St.
Helena, the Cape of Good Hope, Bombay, Madras, Paramatta, Van Diemen's
Island, and with any other instruments likely to be referred to as
standards, or employed in research elsewhere. Any vessel having a
portable barometer on board, the zero of which has been well determined,
would do well, on touching at any of the ports above named, to take
comparative readings with the standards at those ports, and record the
differences between the standard, the portable, and the ship barometers.
By such means the zero of one standard may be transported over the whole
world, and those of others compared with it ascertained. To do so,
however, with perfect effect, will require that the utmost care should
be taken of the portable barometer; it should be guarded as much as
possible from all accident, and should be kept safely in the "portable"
state when not immediately used for comparison. To transport a
well-authenticated zero from place to place is by no means a point of
trifling importance. Neither should it be executed hurriedly nor
negligently. Some of the greatest questions in meteorology depend on its
due execution, and the objects for which these instructions have been
prepared will be greatly advanced by the zero points of all barometers
being referred to one common standard. Upon the arrival of the vessel in
England, at the termination of the voyage, the ship's barometer should
be again compared with the same standard with which it was compared
previous to sailing; and should any difference be found, it should be
most carefully recorded.

The correction for the height of the cistern _above_ or _below_ the