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surd to imagine that these forces should increase ex. actly in proportion to the distances from this axis; for that is an indication of an increase to infinity; whereas the force of attraction is found to decrease in receding from the fountain from whence it flows. But the farther any star is from the quiescent pole, the greater must be the orbit which it describes; and yet it appears to go round in the same time as the nearest star to the pole does. And if we take into consideration the two-fold motion observed in the stars, one diurnal round the axis of the Earth in 24 hours, and the other round the axis of the ecliptic in 25920 years, § 251, it would require an explication of such a perplexed composition of forces, as could

by no means be reconciled with any physical theory. Objec- 112. There is but one objection of any weight tions against

that can be made against the Earth's motion round the the Sun, which is, that in opposite points of the Earth's

Earth's orbit, its axis, which always keeps a paralmotion answered.lel direction, would point to different fixed stars;

which is not found to be fact. But this objection is easily removed, by considering the immense distance of the stars in respect to the diameter of the Earth's orbit; the latter being no more than a point when compared to the former. If we lay a ruler on the side of a table, and along the edge of the ruler view the top of a spire at ten miles distance, and then lay the ruler on the opposite side of the table in a parallel situation to what it had before, the spire will still appear along the edge of the ruler, because our eyes, even when assisted by the best instruments, are incapable of distinguishing so small a change at so great a distance.

113. Dr. BRADLEY found, by a long series of the most accurate observations, that there is a small apparent motion of the fixed stars, occasioned by the aberration of their light, and so exactly answering to an annual motion of the Earth, as evinces the same, even to a mathematical demonstration. Those who are qualified to read the Doctor's modest account of this great discovery, may consult the Philosophical Transactions, No. 406. Or they may find it treated of at large by Drs. Smith*, Longt, DESAGULIERSÍ, RUTHERFURTH||, Mr. MacLAURIN, Mr. SIMPSON), and M. DE LA CAILLE**.

114. It is true that the Sun seems to change his Why the place daily, so as to make a tour round the starry pears to heavens in a year. But whether the Sun or Earth change moves, this appearance will be the same; for, when his place. the Earth is in any part of the heavens, the Sun will appear in the opposite. And therefore this

appearance can be no objection against the motion of the Earth.

115. It is well known to every person who has sailed on smooth water, or been carried by a stream in a calm, that, however fast the vessel goes, he does not feel its progressive motion. The motion of the Earth is incomparably. more smooth and uniform than that of a ship, or any machine made and moved by human art: and therefore it is not to be imagined that we can feel its motion.

116. We find that the Sun, and those planets The on which there are visible spots, turn round their Earth’s axes : for the spots move regularly over their discs. its axis From hence we may reasonably conclude, that demon. the other planets on which we see no spots, and the Earth, which is likewise a planet, have such rotations. But being incapable of leaving the Earth, and viewing it at a distance, and its rotation being smooth and uniform, we can neither see it move

motion on

Optics, B. I. S 1178.

| Astronomy, B. II. S 838. Philosophy, vol. 1. p. 401. W Account of Sir Isaac Newton's Philosophical Discoveries, B. III. C. 2. § 3. Mathemat. Essays, pod.

** Elements d'Astronomie, $ 381,

on its axis as we do the planets, nor feel ourselves affected by its motion. Yet there is one effect of such a motion, which will enable us to judge with certainty whether the Earth revolves on its axis or not. All globes which do not turn round their axes will be perfect spheres, on account of the equality of the weight of bodies on their surfaces; especially of the fluid parts. But all globes which turn on their axes will be oblate spheroids; that is, their surfaces will be higher or farther from the centre in the equatorial than in the polar regions; for, as the equatorial parts move quickest, they will recede far. thest from the axis of motion, and enlarge the equatorial diameter. That our Earth is really of this figure, is demonstrable from the unequal vibrations of a pendulun, and the unequal lengths of degrees in different latitudes. Since then the Earth is higher at the equator than at the poles, the sea, which naturally runs downward, or toward the places which are nearest the centre, would run toward the polar regions, and leave the equatorial parts dry, if the centrifugal force of these parts by which the waters were carried thither did not keep them from return. ing. The Earth's equatorial diameter is 36 miles

longer than its axis, All bodies

117. Bodies near the poles are heavier than those heavier at the poles toward the equator, because they are nearer the than they Earth's centre, where the whole force of the Earth's would be at the

attraction is accumulated. They are also heavier, equator. because their centrifugal force is less, on account

of their diurnal motion being slower. For both these reasons, bodies carried from the poles toward the equator gradually lose of their weight. Experiments prove that a pendulum which vibrates seconds near the poles, vibrates slower near the cquator ; which shews, that it is lighter or less attractive there. To make it oscillate in the same time, it is found necessary to diminish its length. By comparing the different lengths of pendulums

The
Earth's

felt.

swinging seconds at the equator and at London, it is found that a pendulum must be 20lines, or 12th part of an inch shorter at the equator than at the poles. 118. If the Earth turned round its axis in 84 mi. How they

might lose nutes 43 seconds, the centrifugal force would be all their equal to the power of gravity at the equator; and all weight. bodies there would entirely lose their weight. If the Earth revolved quicker, they would all fly off, and leave it.

119. A person on the Earth can no more be sensible of its undisturbed motion on its axis, than one motion in the cabin of a ship, on smooth water, can be sen- cannot be sible of the ship's motion when it turns gently and uniformly round.

It is therefore no argument against the Earth's diurnal motion, that we do not feel it: nor is the apparent revolutions of the celestial bodies every day a proof of the reality of these motions; for whether we or they revolve, the appearance is the very same. A person looking through the cabin-windows of a ship, as strongly fancies the objects on land to go round when the ship turns, as if they were actually in motion.

120. If we could translate ourselves from planet to planet, we should still find that the stars would appear of the same magnitudes, and at the same distances from each other, as they do to us on the Earth, because the diameter of the remotest planet's orbit bears no sensible proportion to the distance of the stars. But then, the heavens would seem to To the difrevolve about very different axes; and consequent-nets the

ferent plaly, those quiescent points, which are our poles in heavens the heavens, would seem to revolve about other appear to points, which, though apparently in motion as seen on differfrom the Earth, would be at rest as seen from any ent axes. other planet. Thus the axis of Venus which lies almost at right angles to the axis of the Earth, would have its motionless poles in two opposite points of the heavens, lying almost in our equi

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noctial, where the motion appears quickest; tecause it is seemingly performed in the greatest circle. And the very poles which are at rest to us, have the quickest motion of all as seen from Venus. To Mars and Jupiter, the heavens appear to turn round with very different velocities on the same axis, whose poles are about 23 degrees from ours.

Were we on Jupiter, we should be at first amazed at the rapid motion of the heavens; the Sun and stars going round in 9 hours 56 minutes. Could we go from thence to Venus, we should be as much surprised at the slowness of the heavenly motions; the Sun going but once round in 584 hours, and the stars in 54.0. And could we go from Venus to the Moon, we should see the heavens turn round with a yet slower motion; the Sun in 708 hours, the stars in 655. As it is impossible these various circumvo. lutions in such different times, and on such different axes, can be real, so it is unreasonable to suppose the heavens to revolve about our Earth, more than it does about any other planet. When we reflect on the vast distance of the fixed stars, to which 162,000,000 of miles, the diameter of the Earth's orbit, is but a point, we are filled with amazement at the immensity of their distance. But if we try to frame an idea of the extreme rapidity with which the stars must move, if they move round the Earth in 24 hours, the thought becomes so much too big for our imagination, that we can no more conceive it than we do infinity or eternity. If the Sun were to go round the Earth in 24 hours, he must travel upward of 300,000 miles in a minute : but the stars being at least 400,000 times as far from the Sun as the Sun is from us, those about the equator must move 400,000 times as quick. And all this to serve no other purpose than what can be as fully and much more simply obtained by the Earth's turning round eastward, as on an axis, every 24 hours; causing thereby an apparent

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