indirectly produces the secondary circulation of Mr. Thompson's
theory. Both these regular movements are, however, greatly disturbed,
and especially the latter, by winds which are occasioned by local
and irregular rains.

In these movements and their causes we have the general outline of
our subject, within which we must now sketch the weather. The causes
of atmospheric movement, which we have thus far considered, are the
unequal distribution of the sun's heat, the absorption and
precipitation of moisture, the direct and the inductive action of
the earth's rotation and friction. If to these we should add the
tidal action of the sun's and moon's attractions, we should perhaps
complete the list of _vera causae_ which are certainly known to
exert a more or less general influence upon the atmosphere. But this
short list is long enough, as we shall soon see.

If the earth were wholly covered with water of a uniform depth, its
climates would be distributed with greater regularity, and the
perturbations of climate would be comparatively small and regular;
though even under such circumstances there would still exist a
tendency to discontinuity and complexity of movements from that
influence of rain, the peculiar character of which we shall soon
consider.

The irregular distribution of land and water, and the peculiar
action of each in imparting the heat of the sun to the incumbent air,--
the irregular distribution of plains and mountains, and their various
effects in different positions and at different altitudes,--the
distribution of heat effected by ocean currents,--all these tend to
produce permanent derangements of climate and great irregularities