icularly
described under the head of accumulations of pressure preceding and
succeeding storms, and minute directions given for the hourly
observations of the necessary instruments. In the mean time we may here
remark that hourly observations under the circumstances above alluded to
are the more important when we consider that the barometer, the
instrument employed in observing these moving atmospheric masses, is
itself in motion. The ship may meet the accumulation of pressure and
sail through it transversely; or she may sail along it, the course of
the vessel being parallel to the line marking the highest pressure, the
ridge or crest of the wave; or the ship may make any angle with this
line: but whatever the circumstances may be under which she passes
through or along with such an accumulation of pressure, it should ever
be borne in mind that her position on the earth's surface is scarcely
ever the same at any one observation as it was at the preceding, the
barometer in the interval has changed _its_ position as well as the line
of maximum pressure, the rate of progress of which it is desirable to
observe. It will, therefore, be at once apparent that in order to obtain
the most accurate data on this head hourly observations are
indispensable. To these readings should of course be appended the places
of the ship from hour to hour, especially if she alter her course much.

There is another point to which we wish to call attention in immediate
connexion with hourly readings--it is the observation of the instruments
on the days fixed for that purpose: they were originally suggested by
Sir John Herschel, whose directions should be strictly attended to: they
are as follows:--

The days fixed upon for these observations are the 21st of March, the
21st of June, the 21st of September, and the 21st of December, being
those, or immediately adjoining to those of the equinoxes and
solstices, in which the _solar influence_ is either stationary or in a
state of most rapid variation. _But should any one of those 21st days
fall on a Sunday, then it will be understood that the observations are
to be deferred till the next day, the 22nd._ The series of observations
on board each vessel should commence at 6 o'clock A.M. of the appointed
days, and terminate at 6 A.M. of the days following, according to the
usual reckoning of time adopted in the daily observations.

In addition to the twenty-five hourly readings at the solstices and
equ