We have seen how the passage of the axis of the vortex may derange the
electric tension of the parts passed over; but there is another mode of
action not yet adverted to.
[Illustration: Fig. 1]
When the moon is at her perigee, the axis of the vortex passes through
the centre of gravity of the earth and moon at C, and cuts off the
segment RR. At the apogee, on account of her greater distance, and of
her consequent power to _push_ the earth out from the axis of the vortex
XX, the segment R'R' is only cut off by the axis; and the angle which
the axis makes with the surface will vary with the arcs AR and A'R'; for
these arcs will measure the inclination from the nature of the circle.
In passing from the perigee to the apogee the axis will pass over the
latitudes intermediate between R and R' in both hemispheres, neither
reaching to the equator E, nor to the pole P. Let us now suppose a
meridian of the earth, represented by the line NRS, N being north, and S
south, and the surface of the atmosphere by N'S'; XX still representing
the axis of the vortex, ordinarily inclined 34d or 35d to the surface.
Let us also conceive the rotation of the earth to cease, (the action of
the vortex remaining the same,) thus leaving the axis over a particular
longitude. If the ether possesses inertia, there will be an actual
scooping out of the upper portions, driving them southward to a certain
distance, where the atmosphere will be piled up above the ordinary
level. There will, therefore, be a strong contrary current at the
surface of the earth to restore the equilibrium, and if the action be
violent, the surface wind will be increased; so that if it be considered
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]