also phænomena that occur in its atmosphere, which require still closer
attention than the eight daily readings. One such portion would appear
to exist off the western coast of Africa, and we recommend the adoption
of _hourly_ readings while sailing to the westward of this junction of
aqueous and terrestrial surface; more attention will be directed to this
point as we proceed. There are also phænomena the localities of which
may be undetermined, and the times of their occurrence unknown, but so
important a relation do they bear to the subject of our inquiries, that
they demand the closest attention. They will be more particularly
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.