Both the student and the reader, are requested to fill out an additional illustration of Jupiter's mean distance.

It will be obvious, that if the true distances from the Sun, of the Earth and of each of the other planets can be ascertained, the nearest approach of each other planet to the Earth, and the remotest distance of each from the Earth can be known.

If Venus is 68 millions of miles from the Sun, and the Earth 95 millions, subtract the 68 from the 95, and you have 27 millions of miles, the distance of Venus in its inferior conjunction. To the distance of the Earth from the Sun, add the distance of Venus from it, and you have 163 millions of miles, the distance of Venus in its superior conjunction.

When the true distance, of each planet from the Earth, is thus ascertained, and their apparent diameters exactly measured, their real magnitudes can be known.

RELATIVE MAGNITUDES.

With a knowledge of their real magnitudes, the proportions in their magnitudes can be estimated; for all spheres, or globes, are in proportion to one another as the cubes of their diameters.

For illustration, let the diameter of the Earth be called in round numbers 8 thousand miles; and the diameter of Jupiter 90 thousand miles; in both cases a trite exceeding their real diameters. The cube of Jupiter's diameter is 90 thousands, multiplied by 90 thousands, and that product by 90 thousands, making 729 millions. The cube of the Earth's diameter is 8 thousand miles multiplied by 8 thousands, and that product by 8 thousands; amounting to 512 thousands, by which divide 729 millions, the cube of Jupiter's diameter, and the result will be 14 hundred; proving that Jupiter has about 14 hundred times the bulk of the Earth.

If the distance of each planet from the Sun is known, how determine their distance from the Earth and their magnitudes? How would you illustrate this?

How can the comparative magnitudes of spheres be known? How would you illustrate this?

"To determine the area of surface in a sphere, multiply the circumference by the diameter.

"To determine the cubical contents of a sphere, multiply the area of surface by 4th of the diameter." WATTS.

Thus we learn, of what incalculable importance to science, are correct observations of a transit of Venus. No wonder, that all ranks and conditions of our race, feel interested in that event.

The governments of Europe, fitted out expeditions to different parts of the Globe in 1761 and 1769, and men of science encountered perils by sea and by land, for the express purpose of observing these transits. And millions of eyes, were intensely gazing on a dark spot crossing the disc of the Sun.

The methods of estimating the densities of the Earth and the heavenly bodies, belong to Physical Astronomy, to which the attention is next to be turned.

CHAPTER XVIII.

PHYSICAL ASTRONOMY.-GRAVITATION.

THE mind may now advance, to that sublime department of science called PHYSICAL ASTRONOMY; which developes the laws of motion, or the uniformity in which their resistible energies of their AUTHOR, govern the mighty movements of material worlds.

That property in bodies, which is manifested in a tendency to approach each other, is called

ATTRACTION.

The magnet attracts the needle. This is called the attraction of magnetism. The feather, suspended near an electrical con

How can the area of surface in a sphere be estimated? How can its cubical contents be determined?

What evidence has been given of the deep interest taken in the transits of Venus?

What is ATTRACTION?

ductor, is attracted by it. This is termed the attraction of electricity. The particles of matter in solid bodies, commonly adhere together in a greater or less degree. This is called the attraction of cohesion. Bodies in general, appear to have a tendency towards each other; this general tendency, is called the ATTRAC

TION OF GRAVITATION.

Indistinct views, of the manner in which the attraction of gravitation is manifested in the material universe, appear to have been possessed by Copernicus, Kepler, Hooke, and others. But the genius of NEWTON, was destined to develope its admirable simplicity and omnipotent energy, and thus immortalize his own name.

Reclining under a fruit tree, and witnessing the fall of an apple the fortunate perhaps entered Newton's mind; 'Perhaps that same principle which made the apple fall extends to the Moon and binds it in its orbit. This mere hypothesis as it was at the first, prepared the way for him, sometime afterwards, to unfold the sublimest truths of the sublimest of the sciences.

Here may be announced the laws of gravitation. Equal quantities of matter, at equal distances, have equal attraction: but attraction diminishes, as the squares of the distances increase.

Some illustration may be useful. Call the distance, from the centre to the surface of the Earth, in round numbers, 4 thousand miles. The Earth's semi-diameter now constitutes an unit, and call that unit one pound weight. Remove that one pound, a second semi-diameter from the Earth's centre, or 4 thousand miles from its surface. Twice two are four. Its tendency to the Earth, would be

what it was at its surface. Remove it another 4 thousand miles, and its tendency would be as great as at the Earth's surface, for 3 times 3 are 9.

What different kinds are there?

By whom were the laws of attraction developed? What are they? How illustrate them?

Remove it to the orbit of the Moon, its tendency would be part as great as at the surface; for the Moon is 60 semi-diameters of the Earth distant from it, and 60 times 60, are 3600.

The semi-diameter of the attracting body, is to be used as an unit in determining the power of its attraction at given dis

tances.

By the laws of gravitation, it is estimated that if no force were felt at the Moon but the attraction of the Earth, it would be drawn to its primary in less than 5 days. Were no force but this centripetal force, acting upon the planets, Mercury would be drawn to the Sun in about 15 days, Venus in 40 days, the Earth in 65 days, Mars in 121 days, the Asteroids in less than 1 year, Jupiter in about 2 years, Saturn in about 5 years, and Herschel in about 15 years, and the Comets from 1 to 100 years.

CHAPTER XIX.

CENTRIFUGAL FORCE.-DENSITIES OF THE SUN AND PLANETS.

IN NATURAL PHILOSOPHY, one essential pro perty of matter is called inertia. That is, if mat ter be supposed to exist in a state of rest, it must forever remain at rest, unless some force from without itself should put it in motion; or if in motion, it would move in a straight line forever, if no force without itself operated to prevent the continuance of such motion. How irresistible then the evidence, that the motions of the material universe were not self-originated; but are the designs of Infinite Wisdom, and the impulses of Omnipotent Goodness. Conceive the Earth, with all its kindred bodies in the solar system, when created, to have

With no counteracting influence, how soon would attraction draw Mercury to the Sun ?-Venus ?-the Earth ?-the other planets?-the comets?

What is inertia? From known properties of matter, what moral conclusion seems unavoidable ?

received from the Almighty Architect only that energy for movement called attraction; how soon must all the planets and comets of this system, have been drawn by their centripetal forces, to one common centre and one entire mass. But that energy which inscribed the law of attraction on material worlds, gave other displays of Omnipotence. A mighty impulse was imparted to the Sun, causing it to turn on its axis and impelling it in space; astronomers know not in how vastly extended an orbit. The same energy, imparted to Mercury a force acting at right angles with the centripetal force which should balance it. This balancing force is called the centrifugal force, because it tends to make the body fly from the centre. And as the attractions of the Sun, are more powerful at 37 millions, than at 95 millions of miles, the centrifugal force, to balance the centripetal, is more powerful than at the Earth, causing Mercury to revolve in its orbit, with the wonderful velocity of 108 thousand miles the hour. See Plate v. Figures 4 and 5.

ap

When the centrifugal force was given to the planets, pears to have been given to the centres of the secondaries, causing one rotation on their axes, while they revolved once round their primaries. But with the primaries, the force seems to have been applied not to their centres, but to their surfaces; giving them double motion, one on their axes, the other round the Sun.

DENSITIES OF THE SUN AND PLANETS.

THE practicability, and methods, of estimating the densities of the Earth, and even of remote worlds, can now be understood.

What is the centripetal force? What the centrifugal force? Why will Mercury revolve with greater velocity than the Earth? How does it appear that the centrifugal force was imparted to secondaries?—to primaries?