every three hours, in order that the undulations arising from the
configuration of the terrestrial and oceanic surfaces may be more
distinctly marked and more advantageously compared with the Gulf of
Mexico, the Caribbean Sea, and the northern portion of the African
continent.


_The Pacific Ocean._--As this ocean presents so vast an aqueous surface,
generally speaking observations at intervals of six hours will be amply
sufficient to ascertain its leading barometric phænomena. Vessels,
however, on approaching the continents of North and South America, or
sailing across the equator, should resort to the three-hourly readings,
in order to ascertain more distinctly the effect of the neighbourhood of
land on the oscillations of the barometer, as generally observed, over
so immense a surface of water in the one case, and the phænomena of the
equatorial depression in the other: the same remarks relative to the
latter subject, which we offered under the head of South Atlantic, will
equally apply in the present instance. The configuration of the western
shores of North America renders it difficult to determine the precise
boundary where the three-hourly series should commence; the 90th
meridian is recommended for the boundary as regards South America, and
from this a judgment may be formed as to where the three-hourly
observations should commence in reference to North America.


In the previous sketch of the localities for the more important
observations, it will be seen that within the tropics there are three
which demand the greatest regard.

I. The Archipelago between the two Americas, more particularly comprised