The Hurricane Guide | Page 9

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
equinoxes as above recommended, it would be desirable to continue the
observations until a complete elevation and depression of the
barometer had been observed at these seasons. This plan is adopted at
the Royal Observatory, Greenwich, and would be attended with this
advantage were it generally so--the progress of the elevation and
depression would be more readily traced and their velocities more
accurately determined than from the four or eight daily readings.
III.--LOCALITIES FOR ADDITIONAL OBSERVATIONS.

In sketching out a system of barometric observation having especial
reference to the acquisition of data from which the barometric character
of certain large areas of the surface of the globe may be
determined--inasmuch as such areas are distinguished from each other,
on the one hand by consisting of extensive spaces of the oceanic
surface unbroken, or scarcely broken, by land; on the other by the
proximity of such oceanic surface to large masses of land, and these
masses presenting two essentially different features, the one consisting
of land particularly characterized as continental, the other as insular,
regard has been accordingly had to such distribution of land and
water.
As these instructions have especial reference to observations at sea,
observations on land have not been alluded to; but in order that the
data accumulated may possess that value which is essential for
carrying on the inquiry in reference to atmospheric waves with success,
provision is made to mark out more distinctly the barometric effects of
the junction of large masses of land and water. It is well known that the
oceanic surface, and even the smaller surfaces of inland seas, produce
decided inflexions of the isothermal lines. They exercise an important
influence on temperature. It has also been shown that the
neighbourhood of water has a very considerable influence in
increasing the oscillations of the mercurial column in the barometer,
and in the great systems of European undulations it is well known that
these oscillations increase especially towards the north-west. The
converse of this, however, has not yet been subjected to observation;
there has been no systematic co-operation of observers for the purpose
of determining the barometric affections of large masses of water, such
as the central portion of the basin of the northern Atlantic, the portion
of oceanic surface between the Cape of Good Hope and Cape Horn,
the Indian and Southern oceans, and the vast basin of the Pacific. Nor
are we yet acquainted with the character of the oscillations, whether
increasing or decreasing, as we recede from the central portions of the
oceanic surfaces we have mentioned towards the land which forms
their eastern, western, or northern boundaries. This influence of the
junction line of land and water, so far as it is yet known, has been kept
in view in framing these instructions, and, as it appears so prominently

in Europe, it is hoped the additional observations between the four
daily readings to which probably many observers may habitually
restrict themselves, making on certain occasions and in particular
localities a series of observations at intervals of three hours, will not be
considered too frequent when the great importance of the problem to
be solved is fully apprehended. It need scarcely be said that the value
of these observations at three-hourly intervals will be greatly increased
by the number of observers co-operating in them. Upon such an
extensive system of co-operation a large space on the earth's surface,
possessing peculiarities which distinguish it from others extremely
unlike it in their general character, or assimilate it to such as possess
with it many features in common, is marked out below for particular
observation, occupying more than two-thirds of a zone in the northern
hemisphere, having a breadth of 40°, and including every possible
variety of