invisible, so the entire length of a comet's tail may be brought into
view, and apparently be formed in a few hours, through some
comparatively slight displacement of the individual meteorites composing
it.

This paradox--for paradox it unquestionably is--affords a curious
illustration of the influence which mathematical power has on the minds
of men. Every one knows that Professor Tait has potential mathematical
energy competent to dispose, in a very short time, of all the
difficulties involved in his theory; therefore few seem to inquire
whether this potential energy has ever been called into action. It is
singular, too, that other mathematicians of great eminence have been
content to take the theory on trust. Thus Sir W. Thomson, at the meeting
of the British Association at Edinburgh, described the theory as
disposing easily of the difficulties presented by Newton's comet in
1680. Glashier, in his translation of Guillemin's 'Les Cometes,' speaks
of the theory as one not improbably correct, though only to be
established by rigid investigation of the mathematical problems
involved.

In reality, not five minutes' inquiry is needed to show any one
acquainted with the history of long-tailed comets that Tait's theory is
quite untenable. Take Newton's comet. It had a tail ninety millions of
miles long, extending directly from the sun as the comet approached him,
and seen, four days later, extending to the same distance, and still
directly from the sun, as the comet receded from him in an entirely
different direction. According to Tait's sea-bird theory, the earth was
at both these epochs in the plane of a sheet of meteorites forming the
tail; but on each occasion the sun also was in the same plane, for the
edge of the sheet of meteorites was seen to be directly in a line with
the sun. The comet's head, of course, was in the same plane; but three
points, not in a straight line, determine a plane. Hence we have, as the
definite result of the sea-bird theory, that the layer or stratum of
meteorites, forming the tail of Newton's comet, lay in the same plane
which contained the sun, the earth, and the comet. But the comet crossed
the ecliptic (the plane in which the earth travels round the sun)
between the epochs named, crossing it at a great angle. When crossing
it, then, the great layer of meteorites was in the plane of the
ecliptic; before crossing it the layer was greatly inclined to that
plane one way, and after crossing