in the college laboratory and who has witnessed a display of Northern
Lights will at once recognize the resemblance between them in colors,
forms, and behavior. This resemblance had often been noted before
Arrhenius elaborated his hypothesis.

Without intending to treat his interesting theory as more than a
possibly correct explanation of the phenomena of the Aurora, we may
call attention to some apparently confirmatory facts. One of the most
striking of these relates to a seasonal variation in the average
number of auroræ. It has been observed that there are more in March
and September than at any other time of the year, and fewer in June
and December; moreover (and this is a delicate test as applied to the
theory), they are slightly rarer in June than in December. Now all
these facts seem to find a ready explanation in the hypothesis of
Arrhenius, thus: (1) The particles issuing from the sun are supposed
to come principally from the regions whose excitement is indicated by
the presence of sun-spots (which accords with Hale's observation that
sun-spots are columns of ionized vapors), and these regions have a
definite location on either side of the solar equator, seldom
approaching it nearer than within 5° or 10° north or south, and never
extending much beyond 35° toward either pole; (2) The equator of the
sun is inclined about 7° to the plane of the earth's orbit, from which
it results that twice in a year -- viz., in June and December -- the
earth is directly over the solar equator, and twice a year -- viz., in
March and September -- when it is farthest north or south of the solar
equator, it is over the inner edge of the sun-spot belts. Since the
corpuscles must be supposed to be propelled radially from the sun, few
will reach the earth when the latter is over the solar equator in June
and December, but when it is over, or nearly over, the spot belts, in
March and September, it will be in the line of fire of the more active