phænomena requiring extraordinary observations for their elucidation:
and _fifth_, to particular seasons, when the instruments should be
watched with more than ordinary care.

The more important objects of observation having especial reference to
atmospheric waves are those points which have been termed _crests_ and
_troughs_. These are simply the _highest_ and _lowest_ readings of the
barometer, usually designated _maxima_ and _minima_, and should for the
object in view receive particular attention. Whenever there is reason to
believe that the barometer is approaching either a _maximum_ or
_minimum_, additional observations should be resorted to, so as to
secure as nearly as possible _the precise time_ as reckoned at the ship,
with her position, of its occurrence, as well as the altitude of the
mercurial column at that time and place. By means of such observations
as these on board several ships scattered over the surfaces of our great
oceans, much valuable information may be accumulated of a character
capable of throwing considerable light on the _direction_ in which the
lines of barometric maxima and minima stretch, and also a tolerably
accurate notion may be formed of their progress, both as regards
direction and rate. In immediate connexion with such observations
particular attention should be paid to the direction of the wind
according to the season.


SECTION I.--INSTRUMENTS.

_Description and Position of Instruments._--The principal instrument
requisite in these observations is the barometer, which should be of the
marine construction, and as nearly alike as possible to those furnished
to the Antarctic expedition which sailed under the command of Sir James