gnetism.

Again, the N. E. trades extended up in summer over the Mediterranean Sea,
an evaporating surface, blow over the Barbary States in June and July, but
furnish no rain. And so of the S. E. or N. E. trades which blow over
Brazil and other countries in the absence north or south of the tropical
belt of rains.

It is obvious from these facts--and more like them might be cited--that
mere evaporation, however copious or long continued, does not make the
storm or shower in the locality where it takes place, and _without the
existence and influential agency_ of a counter-trade; and that _reciprocal
action_, whatever it may be, that takes place _between it and the earth_.

Again, our own experience is conclusive of this. We have no surface-trade
north of 30 deg., and yet a long drought and great evaporation may follow a
wet spring. Belts of droughts and frequent rains occur every year in
different portions of the country side by side, and _the dividing line
follows the course of the counter-trade_, and is sometimes distinctly
marked for weeks. When a change occurs in the counter-trade, whether from
causes existing there or the influence of terrestrial magnetism (in
relation to which we shall inquire hereafter), showers form or storms come
on: until it does they will not. Efforts at condensation will occasionally
appear, but they will be feeble and ineffectual, and occasion a repetition
of the axiom that "all signs fail in a drought." And we may know it from
direct observation.

The first indications of a storm, and of most if not all showers, are
observable in the counter-trade. These indications, so far as they are
visible, are of course to be looked for in the west; although the
direction and character of the surface-winds are often indicative of these
changes when not visible at the west as we shall see.

The indications are those of condensation, and vary very much in different
seasons of the year. It is not my purpose in this place to examine them
particularly. They will be alluded to hereafter under the head of
prognostics. Suffice it now to say, then, that whether it be the long
threads or lines of cirrus which occur in the trade in the winter after a
period of severe cold, following the interposition of a large volume of N.
W. cold air and the elevation of the counter-trade; or the forms of cirrus
which occur at other times and other seasons; or whether it be the
ordinary bank at night-fall, or the evening co