the earth, and with the position of the magnetic pole. It is certain
that the appearance of this meteor does affect the needle in a way not
to be mistaken, and (although not invariably) the vertex of the luminous
arch will usually conform to the magnetic meridian. Yet (and this is
worthy of attention), the observations made in the North Polar
Expeditions[29] "appear to prove, that in the immediate vicinity of the
magnetic pole the development of light is not in the least degree more
intense or frequent than at some distance from it." In fact, as the
American magnetic pole is, as stated, in latitude 73d, the central
vortex will seldom reach so high, and, consequently, the aurora ought
at such times to be more frequent in a lower latitude. In a late work by
M. de la Rive, this gentleman expresses the opinion, that the cause of
the aurora is not due to a radiation of polar magnetism, but to a purely
electrical action.[30] His explanation, however, is not so satisfactory
as his opinion. Now, we have examined numerous cases of auroral
displays, and never yet found one which could not be legitimately
referred to the action of ethereal vortices. Generally, the aurora will
not be visible, when the upper surface of the atmosphere of that
latitude in which the vortex is known to be (reckoning in the direction
of the magnetic meridian) is below the horizon, which shows that the
brightest portion is in the atmosphere. In latitude 41d even, it may
show itself when the vortex is three days north, more frequently when
one or two days north; but when the vortex passes centrally, or south,
it rarely is seen, and this is the only difficulty in explaining it by
the theory. But, when we reflect that the ether shoots out in straight
lines, and at an angle corresponding to the magnetic dip, we are at no
loss to perceive the reason of this. If each minute line composing the
light were seen endwise, it would be invisible; if there were millions
such in the same position, they could add nothing to the general effect;
but, when viewed sideways, the case would be different, there would be a
continued reduplication of ray upon ray, until in the range of some
hundreds of miles an effect might be produced amounting to any degree of
intensity on record. Now, this is the case when the aurora is
immediately overhead, it will be invisible to those below, but may be
seen by persons a hundred miles south; so, also, when it is to the
south, it is too obliq