From patient and protracted investigations, it is estimated, that the mean density of the whole Earth, is about 5 times as great as water; that is, it would weigh five times as much, as if it were composed wholly of pure water.

Numerous and careful experiments were made, on the mountain Schehallien in Scotland, by Dr. Maskelyne, with a plumb line, to determine the attraction of the cubic miles in the mass of that mountain, compared, with the attraction of all the cubic miles of the Earth. The influence of that mountain on the plumb line, led to the conclusion that the density of the Earth is 4 times greater than water. But Maskelyne called the mean density of the mass of matter in Schehallien only 24, while Professor Playfair, has since estimated its mean density at 21, which will make the density of the Earth, 5. Experiments by Cavendish, make it a fraction more than 5. And it is a remarkable fact, that the most recent experiments, best agree with Newton's own theoretical opinions on this subject.

The density of the Earth being known, the degree of attraction it exerts on the Moon at 240 thousand miles distance, requires a centrifugal force to balance it, which causes that satellite to revolve round its primary in 27 days. Jupiter's nearest satellite is about the same distance from its primary as the Moon, but that revolves in 1 day and 18 hours, therefore, the attractions of Jupiter are much stronger than those of the Earth, proving it has more than 300 times as much matter;

What is the estimated density of the Earth?

Who made experiments in Scotland for this purpose? What was his estimated density of the Earth? What did he call the density of Schehallien? What is Professor Playfair's estimate? What infercrence results from it? What is Cavendish's estimate? What Newton's theory?

If with five times the density of water, the Earth attracts the Moon at 240 thousand miles distance, so much as to require a centrifugal force, which should propel it round the Earth in 27 days, how much more matter must there be in Jupiter, since the nearest satellite revolves in 1 day and 18 hours?

but when the bulk of that planet is considered, it is found, that the mean density of Jupiter is only that of the Earth, or 1 that of water.

When the distances and periods of revolution, of Jupiter's other satellites are correctly estimated, each gives the same result in the density of that primary. By a similar process the densities of Saturn and Herschel can be estimated.

It is easy to perceive, that by this process the density of the Sun can be found. The attraction it exerts, in proportion to its magnitude, is balanced by centrifugal forces in the relative velocities of planets, more near, and more remote from it, equal to a density 1 that of water.

The densities of planets which have no satellites, are best learned by accurate observations, of their attractions on other members of the solar system.

CHAPTER XX.

ORBITS OF THE PLANETS.-MUTUAL ATTRACTIONS.

WHEN theories are substituted for a careful investigation of facts, error will be perpetuated, and truth remain undiscovered. Copernicus and Galileo, considered the orbits of all the heavenly bodies perfect circles, the circle being the simplest curve line. Facts have overturned that theory, and proved all their orbits to be elliptical.

Kepler, by a patient observation of the motions of Mars, found they could not be reconciled with that theory. He then supposed the orbit of that planet an ellipse, with the Sun in its centre.

Knowing the bulk of Jupiter, what then must be its density? Will a similar process determine the densities of other planets and of the Sun ?

What was once supposed to be the forms of the orbits of the planets? Who discovered their true forms?

This did not agree with accurate observations of its direct, retrograde and stationary appearances. His next hypothesis was fortunate. He supposed the orbit of Mars an ellipse, with the Sun in one of its foci. Observations confirmed this as truth.

To draw a circle, one leg of the dividers is stationary, the other is turned round it. To draw an ellipse, or oval figure, one end is drawn with a leg stationary at one point, and the other end with a leg stationary at another point. These station. ary points are the foci of that ellipse. The distance between the centre of an ellipse and one of its foci, is called its eccentricity.

Kepler had the honor to discover, that the orbits of ALL the planets are elliptical, having the Sun in their lower focus. Their eccentricities may be seen in the tabular view of the Solar system, page 29.

Mercury's orbit, having an eccentricity of 7 millions of miles, will have its perihelion, 15 millions of miles nearer to the Sun, than its aphelion. The eccentricity of the orbit of Venus, being only half a million of miles, its perihelion varies but 1 million of miles from its aphelion. Addition, or subtraction, of their respective eccentricities, to, or from, their respective mean distances, will give the aphelion, and perihelion, distances of all the other planets.

The eccentricity of the Earth's orbit, is 1 million and a half miles; making its perihelion distance about 3 millions of miles less than its aphelion distance from its centre of motion.

This is proved from the Sun's disc having about 1' greater apparent diameter, the beginning of January, than at the beginning of July; and by the shorter period, which elapses between the autumnal equinox and the vernal, than between the vernal and the autumnal; owing to the Earth's greater velocity in its orbit, when nearest the Sun.

Two forces, which exactly balance each other at right angles, when operating in an ellipse with the centripetal force in one of the foci, will produce

What is an ellipse? What its foci? What its eccentricity? When is a planet in its aphelion? When in its perihelion? How can the elliptical form of the Earth's orbit be proved? What is its eccentricity?

accelerated motion, from the aphelion point to the perihelion point, and retarded motion from the perihelion to the aphelion.

Koler discovered, that lines drawn from each of any two pla ets to the Sun, would pass over equal areas of space in equal times, and that the same is true respecting any planet at This fact gave unequal distances from its centre of motion. hin a key to the proportionate distances, among the planets.

Kindred laws are found to be governing the comets, though moving in orbits vastly more elliptical than the planets. The eccentricities of comet's orbits, vary from 1 million and a half, to more than 6 thousand millions of miles.

MUTUAL ATTRACTIONS.

While the Sun attracts the planets, they attract the Sun. If all the other planets were in a direct line between Herschel and the Sun, that luminary would be attracted, almost the whole of its diameter towards them.

When the motion of one planet, in its orbit, is forward of another, its attraction will accelerate the motion of the planet that is following; but its own velocity will be retarded by the other.

Primary planets, not only affect the motions of each other, and of their own satellites, but they, and their satellites, affect the motions of other

If two forces balance each other at right angles, when will there be accelerated motion?

What other discovery did Kepler make? What is its great utility.

How much does the eccentricity of the orbits of the comets vary? What is said of mutual attractions among the heavenly bodies? When will the motion of a planet be accelerated ?— When will it be retarded?

satellites. More than 50 disturbing attractions, are to be taken into the account, in constructing tables, from which, the Moon's true place, can be known with perfect accuracy.

The Moon's orbit is elliptical, and its attractions, cause the Earth to revolve round their common centre of gravity, 2000 miles distant from the primary. See Plate v. Figure 6.

The Moon's perigeee, and nodes, change their places, and make one complete revolution round the Earth, in something more than 18 years. Some general correspondence, between eclipses, and harvest Moons, will, therefore, occur every 19th year.

CHAPTER XXI.

SPHEROIDAL FORMS OF THE PLANETS.-ZODIAC.CAUSE OF THE PRECESSION OF THE EQUINOXES; AND NUTATION OF THE EARTH'S AXIS.-NEW STYLE. DIMINISHING OBLIQUITY OF THE ECLIPTIC.

When we see a globe in motion, on an axis, it becomes at once apparent, that its equatorial surface, will revolve through a greater portion of space, in a given time, than any intermediate point between that and either pole. Conceive of a globe, called into existence, with so much fluidity of matter, that its form might easily be changed, and let it be set in motion on its axis, it would not remain

How many disturbing influences must be considered to construct tables of the Moon's place with perfect accuracy? What is the form of the Moon's orbit?

How long a period is required for a revolution of the Moon's nodes? What consequence follows?

What is obvious with a globe in motion?