Under this
head we must consider the relation of ascending vapors to the electric
charge and the form of the clouds, according to the different periods of the
day and year; the difference between the cold and warm zones of the earth,
or low and high lands; the frequency or rarity of thunder storms, their
periodicity and formation in summer and winter; the causal connection of
electricity, with the infrequent occurrence of hail in the night, and with
the phenomena of water and sand spouts, so ably investigated by Peltier.
The horary oscillations of the barometer, which in the tropics present two
maxima (viz., at 9 or 9 1/4 P.M., and 4 A.M., occurring, therefore, in
almost the hottest and coldest hours), have long been the object of my most
careful diurnal and nocturnal observations.*
[footnote] *'Observations faites pour constater la Marche des Variations
Horaires du Barometre sous les Tropiques', in my 'Relation Historique du
Voyage aux Regions Equinoxiales', t. iii., p. 270-313.
Their regularity is so great, that, in the daytime especially, the hour may
be ascertained from the height of the mercurial column without an error, on
the average, of more than fifteen or seventeen minutes. In the torrid zones
of the New Continent, on the coasts as well as at elevations of nearly
13,000 feet above the level of the sea, where the mean temperature falls to
44.
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