155. The Orbits of all the Planets are Ellipses, very little different
from Circles: but the Orbits of the Comets are very long Ellipses; the
lower focus of them all being in the Sun. If we suppose the mean
distance (or middle between the greatest and least) of every Planet and
Comet from the Sun to be divided into 1000 equal parts, the
Excentricities of their Orbits, both in such parts and in _English_
miles, will be as follows. Mercury’s, 210 parts, or 6,720,000 miles;
Venus’s, 7 parts, or 413,000 miles; the Earth’s, 17 parts, or 1,377,000
miles; Mars’s, 93 parts, or 11,439,000 miles; Jupiter’s, 48 parts, or
20,352,000 miles; Saturn’s, 55 parts, or 42,735,000 miles. Of the
nearest of the three forementioned Comets, 1,458,000 miles; of the
middlemost, 2,025,000,000 miles; and of the outermost, 6,600,000,000.
[Sidenote: The above laws sufficient for motions both in circular and
elliptic Orbits.]
156. By the above-mentioned laws § 150 _& seq._ bodies will move in all
kinds of Ellipses, whether long or short, if the spaces they move in be
void of resistance. Only, those which move in the longer Ellipses, have
so much the less projectile force impressed upon them in the higher
parts of their Orbits; and their velocities, in coming down towards the
Sun, are so prodigiously increased by his attraction, that their
centrifugal forces in the lower parts of their Orbits are so great as to
overcome the Sun’s attraction there, and cause them to ascend again
towards the higher parts of their Orbits; during which time, the Sun’s
attraction acting so contrary to the motions of those bodies, causes
them to move slower and slower, until their projectile forces are
diminished almost to nothing; and then they are brought back again by
the Sun’s attraction, as before.
[Sidenote: In what times the Planets would fall to the Sun by the power
of gravity.]
157. If the projectile forces of all the Planets and Comets were
destroyed at their mean distances from the Sun, their gravities would
bring them down so, as that Mercury would fall to the Sun in 15 days 13
hours; Venus in 39 days 17 hours; the Earth or Moon in 64 days 10 hours;
Mars in 121 days; Jupiter in 290; and Saturn in 767. The nearest Comet
in 13 thousand days; the middlemost in 23 thousand days; and the
outermost in 66 thousand days. The Moon would fall to the Earth in 4
days 20 hours; Jupiter’s first Moon would fall to him in 7 hours, his
second in 15, his third in 30, and his fourth in 71 hours. Saturn’s
first Moon would fall to him in 8 hours; his second in 12, his third in
19, his fourth in 68 hours, and the fifth in 336. A stone would fall to
the Earth’s center, if there were an hollow passage, in 21 minutes 9
seconds. Mr. WHISTON gives the following Rule for such Computations.
“[31]It is demonstrable, that half the Period of any Planet, when it is
diminished in the sesquialteral proportion of the number 1 to the number
2, or nearly in the proportion of 1000 to 2828, is the time that it
would fall to the Center of it’s Orbit.” This proportion is, when a
quantity or number contains another once and a half as much more.
[Sidenote: The prodigious attraction of the Sun and Planets.]
158. The quick motions of the Moons of Jupiter and Saturn round their
Primaries, demonstrate that these two Planets have stronger attractive
powers than the Earth has. For, the stronger that one body attracts
another, the greater must be the projectile force, and consequently the
quicker must be the motion of that other body, to keep it from falling
to it’s primary or central Planet. Jupiter’s second Moon is 124 thousand
miles farther from Jupiter than our Moon is from us; and yet this second
Moon goes almost eight times round Jupiter whilst our Moon goes only
once round the Earth. What a prodigious attractive power must the Sun
then have, to draw all the Planets and Satellites of the System towards
him; and what an amazing power must it have required to put all these
Planets and Moons into such rapid motions at first! Amazing indeed to
us, because impossible to be effected by the strength of all the living
Creatures in an unlimited number of Worlds, but no ways hard for the
Almighty, whose Planetarium takes in the whole Universe!
[Sidenote: ARCHIMEDES’s Problem for raising the Earth.]
159. The celebrated ARCHIMEDES affirmed he could move the Earth if he
had a place to stand on to manage his machinery[32]. This assertion is
true in Theory, but, upon examination, will be found absolutely
impossible in fact, even though a proper place and materials of
sufficient strength could be had.
The simplest and easiest method of moving a heavy body a little way is
by a lever or crow, where a small weight or power applied to the long
arm will raise a great weight on the short one. But then, the small
weight must move as much quicker than the great weight as the latter is
heavier than the former; and the length of the long arm of the lever to
the length of the short arm must be in the same proportion. Now, suppose
a man pulls or presses the end of the long arm with the force of 200
pound weight, and that the Earth contains in round Numbers
4,000,000,000,000,000,000,000 or 4000 Trillions of cubic feet, each at a
mean rate weighing 100 pound; and that the prop or center of motion of
the lever is 6000 miles from the Earth’s center: in this case, the
length of the lever from the _Fulcrum_ or center of motion to the moving
power or weight ought to be 12,000,000,000,000,000,000,000,000 or 12
Quadrillions of miles; and so many miles must the power move, in order
to raise the Earth but one mile, whence ’tis easy to compute, that if
ARCHIMEDES or the power applied could move as swift as a cannon bullet,
it would take 27,000,000,000,000 or 27 Billions of years to raise the
Earth one inch.
If any other machine, such as a combination of wheels and screws, was
proposed to move the Earth, the time it would require, and the space
gone through by the hand that turned the machine, would be the same as
before. Hence we may learn, that however boundless our Imagination and
Theory may be, the actual operations of man are confined within narrow
bounds; and more suited to our real wants than to our desires.
[Sidenote: Hard to determine what Gravity is.]
160. The Sun and Planets mutually attract each other: the power by which
they do so we call _Gravity_. But whether this power be mechanical or
no, is very much disputed. We are certain that the Planets disturb one
another’s motions by it, and that it decreases according to the squares
of the distances of the Sun and Planets; as light, which is known to be
material, likewise does. Hence Gravity should seem to arise from the
agency of some subtile matter pressing towards the Sun and Planets, and
acting, like all mechanical causes, by contact. But on the other hand,
when we consider that the degree or force of Gravity is exactly in
proportion to the quantities of matter in those bodies, without any
regard to their bulks or quantity of surface, acting as freely on their
internal as external parts, it seems to surpass the power of mechanism;
and to be either the immediate agency of the Deity, or effected by a law
originally established and imprest on all matter by him. But some affirm
that matter, being altogether inert, cannot be impressed with any Law,
even by almighty Power: and that the Deity must therefore be constantly
impelling the Planets toward the Sun, and moving them with the same
irregularities and disturbances which Gravity would cause, if it could
be supposed to exist. But, if a man may venture to publish his own
thoughts, (and why should not one as well as another?) it seems to me no
greater absurdity, to suppose the Deity capable of superadding a Law, or
what Laws he pleases, to matter, than to suppose him capable of giving
it existence at first. The manner of both is equally inconceivable to
us; but neither of them imply a contradiction in our ideas: and what
implies no contradiction is within the power of Omnipotence. Do we not
see that a human creature can prepare a bar of steel so as to make it
attract needles and filings of iron; and that he can put a stop to that
power or virtue, and again call it forth again as often as he pleases?
To say that the workman infuses any new power into the bar, is saying
too much; since the needle and filings, to which he has done nothing,
re-attract the bar. And from this it appears that the power was
originally imprest on the matter of which the bar, needle, and filings
are composed; but does not seem to act until the bar be properly
prepared by the artificer: somewhat like a rope coiled up in a ship,
which will never draw a boat or any other thing towards the ship, unless
one end be tied to it, and the other end to that which is to be hauled
up; and then it is no matter which end of the rope the sailors pull at,
for the rope will be equally stretched throughout, and the ship and boat
will move towards one another. To say that the Almighty has infused no
such virtue or power into the materials which compose the bar, but that
he waits till the operator be pleased to prepare it by due position and
friction, and then, when the needle or filings are brought pretty near
the bar, the Deity presses them towards it, and withdraws his hand
whenever the workman either for use, curiosity or whim, does what
appears to him to destroy the action of the bar, seems quite ridiculous
and trifling; as it supposes God not only to be subservient to our
inconstant wills, but also to do what would be below the dignity of any
rational man to be employed about.
161. That the projectile force was at first given by the Deity is
evident. For, since matter can never put itself into motion, and all
bodies may be moved in any direction whatsoever; and yet all the Planets
both primary and secondary move from west to east, in planes nearly
coincident; whilst the Comets move in all directions, and in planes so
different from one another; these motions can be owing to no mechanical
cause of necessity, but to the free choice and power of an intelligent
Being.
162. Whatever Gravity be, ’tis plain that it acts every moment of time:
for should it’s action cease, the projectile force would instantly carry
off the Planets in straight lines from those parts of their Orbits where
Gravity left them. But, the Planets being once put into motion, there is
no occasion for any new projectile force, unless they meet with some
resistance in their Orbits; nor for any mending hand, unless they
disturb one another too much by their mutual attractions.
[Sidenote: The Planets disturb one another’s motion.