new, successive field, enables them to maintain and extend
their action.

The following diagram illustrates the course and gradual enlargement of a
mid-autumn tropical storm, which induces a S. E. wind in front, and
occasions a thaw.

[Illustration: Fig. 20.]

2. Another class originate at the N. W., and extend gradually south
easterly on the magnetic meridian. These are most frequent in summer,
forming belts of showers, but occur, I believe, at all seasons of the
year. They seem to be produced by magnetic waves passing south, and are
followed in autumn and winter, and sometimes in summer, by the peculiar N.
W. wind and scud, and a term of cooler weather.

Thus, it is believed that many, perhaps all of the alternating terms of
heat and cold, are dependent on magnetic waves passing over the country in
a similar manner, with a greater or less belt of condensation between
them, and depending on peculiar magnetic action traveling in the same
way. The S. E. extension of showers and storms, and the cooler changes of
temperature which immediately follow them; with light N. W. wind in
mid-summer, and with it fresher at earlier and later periods, in the form
of northers blowing violently, according to the season, are intimately
connected, and indicate such waves. The indication is strengthened also by
the frequent progress of auroras in like manner, occurring usually after
the belt of condensation has passed, and frequently following it. The
clouds and currents of the atmosphere, so far as I have been able to
discover, show no permanent current from the pole to the atmospheric
equator, compensating for the counter-trade; and that compensation is
furnished by the periodical but frequent atmospheric waves, connected with
the periodical changes of storm, and cloud, and sunshine, which gradually
extend from north to south, in or near the magnetic meridian. Perhaps such
compensating currents are found west of the magnetic poles, as we have
suggested, and make the N. E. and northerly dry winds of Western Europe
and the Pacific; but, in the present state of our knowledge, it is
impossible to say that they are. If it be so, the compensation they
furnish must be small; for the volume of counter-trade which is not
depolarized before it reaches the Arctic circle, and which passes round
the magnetic pole, must be very small. A majority of our periodical
changes, during the northern transit, and I believe at all seasons, are of
this