San Andreas independent., 1856-10-11

~ paw VI al OpJect, 0cca
stoned by the real change in the place o
the spectator. Since the parallactic mo
tlon of the forest trees becomes less anc
less perceptible as velocity of the trav
cling beholder diminishes, or as the dis
tance OF the seemingly moving object be.
comes greater, it is evident that to measure
the distance of the fixed stars is eouiva
lent to determining the amount of the
parallatic change in their relative positions
occasloned by the actual change of the DO.
sltlons from which they may be viewed by
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
1t was asserted to revolve in an ellipse of
nearly six hundred million miles in circum-
ference, and with motion so swift that it
passed over no less than sixty-eight thou
sand miles in every hour of time the op-
ponents of the great philosopher exclaimed,
that this doctrine could not be true; 'sfor, y
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 satisfac-
tory reply. They could only venture a
suggestion that owing perhaps to the enor-
nous distance of the fixed stars, no per-
ceptlble change was operated by the revo-
luton of the earth in its orbit in other
words, that the pole of the heavens re-
volved in 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 contest of three hundred
years duration, the truth uttered by Co.
pernicus, but not sufficiently illustrated, is
at length indisputably established
Sometimes things of grosser sort will
serve to make those of finer quality not
only more appreciable, but more intelligi-
BILE. Questions in regard to the subtle
essence Light are difficult because of their
fneness; but it has been found possible to
make them clear by resembling the SUBJ
jects they regard to tangible objects, such
as gun-boats, and riae-balls, and gun bar
rels. One of the last-named articles is
supposed to be placed on moving boat,
and it is proposed so to direct a rife on
shore as to fire ball down the said bar
rel. Now, let the two rifles be on the
same exact level, and the axes of the bar
rels be made precisely to coincide-would
the ball from the one pass down the other,
IN case the fixed one were fired at the ex-
act instant the muzzles came precisely op-
posite to each other? The uninstructed
would be apt to answer Yes; while the
scientific would very confidently reply,
No; it is necessary that the fixed rife
should be fired before the moving one
comes opposite, and the rifeman must
make an allowance for the time the ball re-
quires to move from the one gun to the
other, and also for the velocity with which
the moving piece is descendingth the stream
IN order so that the ball from the shore
may be caused to enter the muzzle of
the moving rife, this computation must
be accurately made. But further con-
ditions 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 oder to avoid the pressure of
the ball against the upper side of the bar
rel, the latter must be fixed ina inclined
position, and that the bottom of the barrel
must be as far up the stream as it will de-
scend by the boats motion during the
progress of the ball down the barrel; in
fine, that the direction in which the bar
rel of the rife 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 rife.
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 rife.
The gun-barrel on the moving boat repre-
sents 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 rife-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 deter
mined, the velocity of light itself becomes
known from these two data and thus the
previously determined value of this incred-
ible velocity is satisfactorily confirmed
For the rest, the reality of the earth S
motion is absolutely necessary to render
the phenomena at all explicable. Such
an illustration may serve to explain to the
grossest understanding how it is that,
owing to the progressive motion of light,