d
tangential to the surface at S, its own momentum will tend to make it
leave the surface and mount up to T; and in this way increase the action
due to the ether. Now, although the axis is never stationary, but
travels round the earth in less than twenty-five hours, yet there is a
tendency to this mode of action; and it is even sometimes palpable to
the observer when the axis has passed immediately to the northward; for
the pinnate shafts and branching plumes of the cirri often reach far to
the south of the southern boundary of the storm. These shafts are always
longer when radiating from the northward than when proceeding from the
southward. The cause is understood by the above figure. At such a time,
after dark, the auroral shafts will also be seen over the storm to the
northward, but will be invisible to those beneath. There is this to be
observed, however, that the visibility of the ethereal current (or the
aurora) is more frequent when the passage of the vortex is not attended
with any great commotion, its free passage being perhaps obstructed by
too dry an atmosphere; hence it becomes more visible. But it may be
asserted that a great aurora is never seen except when a vortex is near,
and to the northward, and within a few hours of its passage over the
meridian. We have, however, seen partial auroras to the south when none
existed north, and also cases when the radiation was from west, but they
are never as bright as in the north. They are all due, however, to the
same cause; and we have frequently followed a vortex for three days to
the northward, (that is, seen the effects of its meridian passage,) at
700 miles distance, by the aurora, and even by the lightning, which
proves plainly that the _exterior layers_ of our atmosphere can reflect
a flash of lightning, assisted by the horizontal refraction, otherwise
the curvature of the earth would sink it ten miles below the horizon.

[Illustration: Fig. 2]

LIMITS OF THE VORTEX.

The action of the polar current of the ether, therefore, tends to cause
a depression of the barometer, and an elevation to the _northward_ and
southward, and there is a general set of the wind below to the point of
greatest depression. The action of the tangential current works the
outer surface of the atmosphere into great ridges and hollows, whose
distances apart as well as actual dimensions, are continually changing
under the influences of causes not yet alluded to, and it is in the
ho