bubble gently blown aside, without detaching it from the pipe, will
afford a good illustration of the mode, and a confirmation of the cause.
The angles measured by Struve, reckoned from the radius vector,
prolonged towards the sun, are subjoined:

November 7 99d.7 | December 7 154d.0
November 30 145 .3 | December 14 149 .4

At this last date, the comet was getting pretty close to the sun. When
the angle was greater, as on November 7th, the comet appeared to make
almost a right angle with the radius vector; and in this position of the
earth and comet, the longer axis of the elliptical comet was directed to
the axis of the vortex, as may be verified by experiment. At the later
dates, the comet was more rapidly descending, and, at the same time, the
axis of the comet was getting more directed towards the earth; so that
the angle increased between this axis and the radius vector, and
consequently became more coincident with it. We have now to consider the
luminous appendage of a comet, commonly called a tail.

The various theories hitherto proposed to account for this appendage are
liable to grave objections. That it is not refracted light needs not a
word of comment. Newton supposes the tail to partake of the nature of
vapor, rising from the sun by its extreme levity, as smoke in a chimney,
and rendered visible by the reflected light of the sun. But, how vapor
should rise towards opposition in a vacuum, is utterly inexplicable. In
speaking of the greater number of comets near the sun than on the
opposite side, he observes: "Hinc etiam manifestum est quod coeli
resistentia destituuntur."[46] And again, in another place, speaking of
the tail moving with the same velocity of the comet, he says: "Et hinc
rursus colligitur spatia coelestia vi resistendi destitui; utpote in
quibus non solum solida planetarum et cometarum corpora, sed etiam
rarissimi candarum vapores motus suos velocissimos liberrime peragunt ac
diutissime conservant." On what _principle_, therefore, Newton relied to
cause the vapors to ascend, does not appear. Hydrogen rises in our
atmosphere because specifically lighter. If there were no atmosphere,
hydrogen would not rise, but merely expand on all sides. But, a comet's
tail shoots off into space in a straight line of one hundred millions of
miles, and frequently as much as ten millions of miles in a single day,
as in the case of the comet of 1843. Sir John Herschel observes, tha