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Page 29
By holding a paper near the eye-piece, the proper direction of
the instrument may be discovered without injury to the eyes. By
this means the sun can be studied from day to day, and its spots or
the transits of Mercury and Venus shown to any number of spectators.
[Illustration: Fig. 37.--Holding Telescope to see the Sun's Spots.]
First covering the eyes with very dark or smoked glasses, erect
a disk of pasteboard four inches in diameter between you and the
sun; close one eye; stand near it, and the whole sun is obscured.
Withdraw from it till the sun's rays just shoot over the edge of
the disk on every side. Measure the distance from the eye to the
disk. You will be able to determine the distance of the sun by
the rule of three: thus, as four inches is to 860,000 miles, so
is distance from eye to disk to distance from disk to the sun.
Take such measurements at sunrise, noon, and sunset, and see the
apparently differing sizes due to refraction.
[Page 97]
VI.
THE PLANETS, AS SEEN FROM SPACE.
"He hangeth the earth upon nothing."--_Job_ xxvi. 7.
[Page 98]
"Let a power be delegated to a finite spirit equal to the projection
of the most ponderous planet in its orbit, and, from an exhaustless
magazine, let this spirit select his grand central orb. Let him with
puissant arm locate it in space, and, obedient to his mandate, there
let it remain forever fixed. He proceeds to select his planetary
globes, which he is now required to marshal in their appropriate
order of distance from the sun. Heed well this distribution; for
should a single globe be misplaced, the divine harmony is destroyed
forever. Let us admit that finite intelligence may at length determine
the order of combination; the mighty host is arrayed in order.
These worlds, like fiery coursers, stand waiting the command to
fly. But, mighty spirit, heed well the grand step, ponder well
the direction in which thou wilt launch each wailing world; weigh
well the mighty impulse soon to be given, for out of the myriads
of directions, and the myriads of impulsive forces, there comes
but a single combination that will secure the perpetuity of your
complex scheme. In vain does the bewildered finite spirit attempt
to fathom this mighty depth. In vain does it seek to resolve the
stupendous problem. It turns away, and while endued with omnipotent
power, exclaims, 'Give to me infinite wisdom, or relieve me from
the impossible task!'"-0. M. MITCHEL, LL. D.
[Page 99]
VI.
_THE PLANETS, AS SEEN FROM SPACE_
If we were to go out into space a few millions of miles from either
pole of the sun, and were endowed with wonderful keenness of vision,
we should perceive certain facts, viz: That space is frightfully
dark except when we look directly at some luminous body. There is
no air to bend the light out of its course, no clouds or other
objects to reflect it in a thousand directions. Every star is a
brilliant point, even in perpetual sunshine. The cold is frightful
beyond the endurance of our bodies. There is no sound of voice in
the absence of air, and conversation by means of vocal organs being
impossible, it must be carried on by means of mind communication.
We see below an unrevolving point on the sun that marks its pole.
Ranged round in order are the various planets, each with its axis
pointing in very nearly the same direction. All planets, except
possibly Venus, and all moons except those of Uranus and Neptune,
present their equators to the sun. The direction of orbital and
axial revolution seen from above the North Pole would be opposite
to that of the hands of a watch.
[Illustration: Fig. 38.--Orbits and Comparative Sizes of the Planets.]
The speed of this orbital revolution must be proportioned to the
distance from the sun. The attraction of the sun varies inversely
as the square of the distance. [Page 100] It holds a planet with a
certain power; one twice as far off, with one-fourth that power.
This attraction must be counterbalanced by centrifugal force; great
force from great speed when attraction is great, and small from less
[Page 101] speed when attractive power is diminished by distance.
Hence Mercury must go 29.5 miles per second--seventy times as fast
as a rifle-ball that goes two-fifths of a mile in a second--or be
drawn into the sun; while Neptune, seventy-five times as far off,
and hence attracted only 1/5626 as much, must be slowed down to 3.4
miles a second to prevent its flying away from the feebler
attraction of the sun. The orbital velocity of the various planets
in miles per second is as follows:
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