Chapter III: Observations

In sketching out a system of observation having especial reference to atmospheric waves and rotatory storms, regard has been had--_first_, to the instruments that should be used, the observations to be made with them, the corrections to be applied to such observations, and the form of registry most suitable for recording the results: _second_, to the times of observation: _third_, to the more important localities that should be submitted to additional observation: _fourth_, to peculiar 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.