Sidereal revolution, 84 y. 8 d. 9 h. 33 m.

Place of the ascending node, Gemini, 12° 59′ 4′′. Place of the descending node, Sagittarius, 12° 59′ 4′′. Motion of the nodes in longitude for 100 years, 26' 10". Retrograde motion of the nodes in 100 years, 57′ 22′′. Place of the aphelion, 11 s. 17° 48′ 6′′.

Motion of the aphelion in longitude for 100 years, 1° 28' 0".

Mean distance of the planet from the Sun, 1800,000,000 miles.

Eccentricity, 86,263,800 miles.

SATELLITES OF HERSCHEL.

Six satellites have been discovered, accompanying Herschel in his dark and tedious round. "It is remarkable," says Prior, "that these satellites revolve in a retrograde direction, or contrary to the order of the signs, in orbits lying nearly in the same plane, and almost perpendicular to the plane of the planet's orbit." This statement is corroborated by other accounts. The satellites of Herschel were all discovered by Dr. Herschel.

When was the planet Herschel discovered? By whom? If it had been before seen, what had it been considered? How was Dr. Herschel employed when he discovered this planet? To whom did it first occur that it was a planet? Why did Dr. Herschel call it Georgium Sidus? How does it appear to the naked eye? Has its diurnal rotation been determined? Why is Herschel denoted by this character, H? How many satellites has Herschel? What is remarkable in the motion of these satellites?

CHAPTER II.

Causes of the Planetary Motion.

MATTER is in itself inactive, and moves but as impelled by external force. An impulse being given to a body, it passes in a right line, till turned out of its course by a different impulse, not in direct coincidence or opposition to the former. Uninterrupted, it would forever move in the same direction, and at the same rate, or over equal distances in equal times. After every new impulse, it will take a new direction, and pass in a diagonal between its former course and the direction of the new impulse. Let the body at A, [Plate v. Fig. 9,] be impelled by a momentum sufficient to carry it in a given time from A to B. It would, uninterrupted, move from B to C, and from C to D, equal distances in equal times. But if, at B, it receive an impulse in the direction BE, sufficient to carry it to E in the same time that the former motion would carry it to C, it would move in the diagonal B F, and be found at F at the same time that it would have arrived at C, unaffected by the impulse last given.-See Enfield's Philosophy, Book II. Chap. iii. Proposition 14.

Circular or elliptical motion is the effect, not merely of an impulse in one direction,) but of such an impulse and a continued action forcing a body from a right line towards a centre. The planets all move in ellipses, differing, however, but little from circles, except the orbits of Juno and Pallas. They are kept in their orbits by the projectile force given at their formation by the Creator, and the constant force of gravity, or the Sun's attraction. Let A, a planet, [Plate vi. Fig. 1,] be projected along the line A B C, meeting with no resistance, it would forever retain the same velocity, and the same direction. The force, which would carry it from A to B

76

CAUSES OF THE PLANETARY MOTION.

in a given time, would, in an equal time, carry it from B to C, and from C to D. But if, at B, it fall into the attraction of S, the Sun, which should so balance the projectile force as to carry it to E, at the same time that it would, by its former motion, have arrived at C, the planet would now revolve in a circle, B E F. But should the attraction of S be more powerful in proportion to the projectile force, it might bring the planet to G instead of E, or, being stronger, might carry it nearer the line B S, in any given proportion. If carried to G, it would revolve in the ellipse B G H. Before it arrives at G, and for some distance after, the lines of motion, caused by the projectile and centripetal forces, form an acute angle. The two powers, then, augment the motion caused by each other; and, attraction increasing as the squares of the distances decrease, the motion of the planet would be accelerated all the way from B to H. At H, it would be nearer the centre of attraction than at B, by twice the eccentricity of its orbit, and, being much more powerfully attracted, would be drawn to S, were not the projectile force also increased. This would now be so augmented, that it would carry the planet from H to I in the same time that attraction would bring it to S. It would then be found at L, and proceed to B, completing the revolution. In passing from F to B, the planet would be as much retarded in its motion by gravity, as accelerated in its motion from B to F.

Thus it appears "that 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." Gravity in one part of an orbit becomes jectile force in another. What is gravity at B is projectile force at M. It operates principally by oblique action. This is immensely increased all the way from B to M.

pro

A double projectile force will always balance a (quadruple power of gravity. The projectile force is greater

in proportion to the centripetal, as the orbit is larger in which the planet moves. In the annual revolution of the Earth, if an angle of one degree be taken at the centre, the projectile force is to the centripetal about as 103 to 1. The disproportion is still greater in the orbits of the superior planets; greatest in that of Herschel.

The solar attraction at Herschel is 3,240,000,000,000,000,000 times less than it is one mile from the Sun. To us it is inconceivable, that such a diminished attraction can have any perceptible effect at the Geor-.. gium Sidus. It must, however, be remembered, that the planets move without resistance in an empty void. That bodies, balanced as they are, can be moved by a very small force, cannot be doubted. Attraction withdrawn, we know not but Archimedes could have made good his assertion by the force of his hand. That bodies so immense, however, and so immensely distant from each other, should all revolve in perfect harmony, may well excite the admiration of limited mortals; but must be infinitely easy to Almighty Power that, at the creation, "spake, and it was done."

How is matter in itself? If a body be put in motion by an impulse, how would it pass? Of what is circular or elliptical motion the effect? In what figures do the planets move? How are they kept in their orbits? What does gravity in one part of an orbit become in another? What will a double projectile force balance? Does the proportion of the projectile force increase as the orbit is larger? In what orbit is the proportion of the projectile force greatest? Can the solar attraction have any perceptible effect at Herschel? If attraction were withdrawn, what is it possible Archimedes could have done? What power keeps the orbits duly balanced?

"Give me where I may stand, and I will move the Earth." This was applied by the celebrated Syracusan to the mechanical force of the lever, but may be true in respect to the movement of the planets in empty void.

CHAPTER III.

Prospect of the Heavens, as seen from different Parts of the Solar System.

SECTION I. Prospect at the Sun.

AT the centre only can a just view be had of the solar system. Stationed there, a spectator would see all the primary planets moving in harmonious order from west to east, He might take the periodical time of one, perhaps Mercury, by which to measure the revolutions of the rest. From their periodical times) and apparent diameters, he might form some conjecture of their distances and magnitude. As all do not move in the same orbits, their paths would appear to cross each other at very small angles. The orbits being elliptical, but the line from any planet to the spectator passing equal areas in equal times, the spaces measured by such planet in a given time, would (appear unequal. Some variation in the diameter of each planet would also be apparent. The fixed stars would appear equally distant, and all at rest.)

Where only can a just view be had of the solar system? Stationed at the Sun, how would a spectator see the primary planets move? By the periodical time of what planet might he measure the revolutions of the rest? From what could he form a conjecture of the distances and magnitude of the planets? Would the spaces measured by a planet in a given time appear equal? How would the fixed stars appear?

SECTION II. Prospect at Mercury.

A different view would be presented at Mercury. A spectator at this planet being, by the whole diameter