or ions, driven away from the sun by light-pressure -- a hypothesis
which seems to explain so many things -- and offers it also as an
explanation of the way in which the sun creates the Aurora. He would
give the Aurora the same lineage with the Zodiacal Light. To
understand the application of this theory we must first recall the
fact that the earth is a great magnet having its two opposite poles of
magnetism, one near the Arctic and the other near the Antarctic
Circle. Like all magnets, the earth is surrounded with ``lines of
force,'' which, after the manner of the curved rays we saw in the
photograph of a solar eclipse, start from a pole, rising at first
nearly vertically, then bend gradually over, passing high above the
equator, and finally descending in converging sheaves to the opposite
pole. Now the axis of the earth is so placed in space that it lies at
nearly a right angle to the direction of the sun, and as the streams
of negatively charged particles come pouring on from the sun (see the
last preceding chapter), they arrive in the greatest numbers over the
earth's equatorial regions. There they encounter the lines of magnetic
force at the place where the latter have their greatest elevation
above the earth, and where their direction is horizontal to the
earth's surface. Obeying a law which has been demonstrated in the
laboratory, the particles then follow the lines of force toward the
poles. While they are above the equatorial regions they do not become
luminescent, because at the great elevation that they there occupy
there is virtually no atmosphere; but as they pass on toward the north
and the south they begin to descend with the lines of force, curving
down to meet at the poles; and, encountering a part of the atmosphere
comparable in density with what remains in an exhausted Crookes tube,
they produce a glow of cathode rays. This glow is conceived to
represent the Aurora, which may consequently be likened to a gigantic
exhibition of vacuum-tube lights. Anybody who recalls his student days