the Pleiades, now supposed to occupy the
centre of gravity, and to be at present the
sun about which the universe of stars composing
our astral system are all revolving;
the light from Alcyone requiring a period of
five hundred and thirty-seven years to traverse
the distance of the sun, from the central
orb about which he performs his mighty
revolutions; and the enormous term of eighteen
million two hundred thousand years
being required to be accomplished, if we may
rely on the angular motion of the sun and
system, as already determined, before the
solar orb, with all its planets, satellites, and
comets will have completed one revolution
around its grand centre.

Still keeping to the incidents of travel, and
the phenomenon of forest trees. Who has
not observed, while journeying along a railway,
how the trees of a forest apparently
whirl around each other—an appearance produced
by the rapid speed of the carriage?
This incident, familiar as it is, may serve to
raise habitually in the mind the notion of the
parallax of the fixed stars. Parallax is the
apparent change in the place of an object,
occasioned by the real change in the place of
the spectator. Since the parallactic motion
of the forest trees becomes less and less perceptible
as the velocity of the travelling beholder
diminishes, or as the distance of the
seemingly moving object becomes greater, it
is evident that to measure the distance of the
fixed stars is equivalent to determining the
amount of the parallactic change in their relative
positions, occasioned by the actual change
of the positions from which they may be
viewed by a spectator on the earth's surface.
The spectator will, on the prompting of this
remarkable suggestion, probably remember
that when the orbitual motion of the earth
was first propounded by Copernicus, and it
was asserted to revolve in an ellipse of nearly
six hundred million miles in circumference,
and with a motion so swift that it passed
over no less than sixty-eight thousand
miles in every hour of time, the opponents
of the great philosopher exclaimed,
that this doctrine could not be true; "for,"
said they, "if we are sweeping around the
sun in this vast orbit, and with this amazing
velocity, then ought the fixed stars to whirl
round each other, as do the forest trees to the
traveller flying swiftly by them." To the
unassisted eye this, which was the case in
fact, did not appear; and the Copernicans
were without a satisfactory reply. They
could only venture a suggestion that, owing
perhaps to the enormous distance of the fixed
stars, no perceptible change was operated by
the revolution of the earth in its orbit; in
other words, that the pole of the heavens
revolved in a curve of two hundred million
miles in diameter, but that such was
the distance of the spheres of the fixed
stars, that this curve was reduced to an
invisible point. After a contest of three
hundred years' duration, the truth uttered by
Copernicus, but not sufficiently illustrated, is
at length indisputably established.

Sometimes things of a grosser sort will
serve to make those of a finer quality not
only more appreciable, but more intelligible.
Questions in regard to the subtle essence,
Light, are difficult because of their fineness;
but it has been found possible to make them
clear by resembling the subjects they regard
to tangible objects, such as gun-boats, and
rifle-balls, and gun-barrels. One of the last-
named articles is supposed to be placed on
a moving boat, and it is proposed so to
direct a rifle on shore as to fire a ball
down the said barrel. Now, let the two
rifles be on the same exact level, and the
axes of the barrels be made precisely to
coincide,—would the ball from the one
pass down the other, in case the fixed one
were fired at the exact instant the muzzles
came precisely opposite to each other? The
uninstructed would be apt to answer yes; not
because the scientific reply confidently, No.
It is necessary that the fixed rifle should be
fixed before the moving one comes opposite,
and the rifleman must make an allowance for
the time the ball reques to move from the
one gun to the other, and also for the velocity
with which the moving piece is descending the
stream. In order that the ball from the shore
may be caused to enter the muzzle of the
moving rifle, this computation must be accurately
made. But further conditions have
also to be considered. For instance, it must
be recollected that while the ball is progressing
down the barrel, the barrel itself is progressing
down the tide, and that, in order to
avoid the pressure of the ball against the
upper side of the barrel, the latter must be
fixed in an inclined position, and that the
bottom of the barrel must be as far up the
streat as it will descend by the boat's motion
during the progress of the ball down the
barrel;—in fine, that the direction in which
the barrel of the rifle which should receive
the ball must be placed, is determined both
by the velocity of the ball, and the velocity of
the boat which bears the rifle.

But what has this very material parable to
do with the theory and properties of light?
First of all, we liken the particles of light
that are shot from the fixed stars to the balls
that are shot from the fixed rifle. The gun-
barrel on the moving boat represents the tube
of the star-gazer, and the boat represents the
earth which bears him while itself sweeping
around in its orbit. Down the axis of that
tube the particles of light, like the aforesaid
rifle-balls, must pass, in order to reach the
eye of the observer. As the velocity of the
earth's motion has been ascertained, and as
the amount by which the telescope must be
inclined, to cause the light to enter, has been
determined, the velocity of the light itself
becomes known from these two data; and
thus the previously determined value of this