The Sun and planets mutually attract each other, the

power by which they are thus attracted, is called Gravity. But whether this power be mechanical or not, is very much disputed. Observation proves that the planets disturb each other's motions by it, and that it decreases according to the squares of the distances of the Sun and planets, as great light which is supposed 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 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 magnitudes or quantities of surface, acting as freely on their internal as external parts, it appears to surpass the powers of mechanism, and to be either the immediate agency of the Deity, or affected by a law originally established and impressed on all matter by him. That the projectile force was at first given by the Deity, is evident; since matter can never put itself in motion, and all bodies may be moved in any direction whatever, and yet the planets, both primary and secondary, move from west to east, in lines nearly coincident, while the Comets move in all directions, and in planes very different from each other; these motions can be owing to no mechanical cause or necessity, but to the free will and power of an intelligent Being.

Whatever Gravity be, it is plain that it acts every moment of time; for if its action should 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 in 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 each other too much by their mutual attraction.

It is found that there are disturbances among the planets in their motions, arising from their mutual attractions, when they are in the same quarter of the Heavens, and the best modern observers find that our years are not always precisely of the same length.

If the planets did not mutually attract each other, the areas described by them would be exactly proportional to the times of description. But observations prove that these areas are not in such exact proportions, and are most varied when the greatest number of planets are in any particular quarter of the Heavens. When any two planets are in conjunction, their mutual attractions which tend to bring them nearer to each other, draws the inferior one a little nearer to him; by these means, the figure of their orbits is somewhat altered, but this alteration is too small to be discovered in several ages. By the most simple law, the diminution of Gravity, as the square of the distance increases, the planets are not only retained in their orbits, when whirling with such immense velocity around their central Sun; but an eternal stability is insured to the solar system. The small derangements which affect the motions of the Heavenly bodies, are only apparent, to the eye of the Astronomer, and even these, after reaching a certain limit, gradually diminish, till the system, regaining its balance, returns to that state of harmony and order which has preceded the commencement of these secular irregularities. Even amidst the changes and irregularities of the system, the general harmony is always apparent; and those partial and temporary derangements, which to vulgar minds may seem to indicate a progressive decay, serve only to evince the stability and permanency of the whole.

In contemplation of such a scene, every unperverted mind must be struck with that astonishing wisdom which framed the various parts of the Universe, and bound them together by one simple, yet infallible law. In no part of creation, from the smallest insect to the highest seraph, has the Supreme Architect of the Universe left himself without a witness; but it is surely in the Heavens above, that the Divine attributes are most gloriously displayed.

INTERROGATIONS FOR SECTION FIFTH.

From what source do the circular motions of the planets arise?

With what velocity would projected bodies continue to move if they met with no resistance? What is meant by the centrifugal force ? What is meant by centripetal force?

Are the centripetal and centrifugal forces ever equal while the planet performs its revolution round the Sun ? When the power of Gravity exceeds the projectile force, why does it not draw the planets to the Sun? When the projectile, or centrifugal force exceeds the attraction, why does it not fly off, and never return? When these forces are equal, why do they not move in perfect circles ?

What will a double projectile force balance?

In what case could the projectile be made to balance the gravitating power in such manner that the planets should move in a perfect circle ?

If the centrifugal forces should at once be destroyed, in what time would each of the planets fall to the Sun? In what time would the Moon fall to the earth?

What rule for finding the time in which they would fall to the Sun ?

What do the rapid motions of the Moons of Jupiter and Saturn demonstrate ?

Do the Sun and planets continually attract each other? Should gravity instantaneously cease, what would be the consequence?

Are the motions of the planets continually the same? Do they continue to move exactly in the same path at every revolution?

By what simple law does gravity diminish?

SECTION SIXTH.

ON LIGHT AND AIR.

LIGHT consists of exceedingly small particles of matter, issuing from a luminous body, as from a lighted candle. Such particles of matter constantly flow in every direction. By Dr. Neiwentyt's computation, 148,660,000,000,000,000,000,000,000,000,000,000,000,000,000 particles of light in one second of time flows from a candle, which number contains at least 6,337,245,000,000 times the number of grains of sand in the whole earth; supposing 100 grains of sand to be equal in length to an inch, and consequently every cubic inch of the earth to contain one million of such grains. These amazingly small particles, by striking upon our eyes, excite in our minds the idea of light, and if they were as large as the smallest particle of matter discernible by our best microscopes, instead of being serviceable to us, they would soon deprive us of sight by the force arising from their immense velocity, which is computed at nearly two hundred thousand miles in one second.*

When these small particles flowing from a candle, fall upon bodies, and are thereby reflected to our eyes, they excite in us the idea of that body, by forming its image on the retina. Since bodies are visible on all sides, light must be reflected from them in all directions. A ray of light is a continued stream of these particles, flowing from any visible body in a straight line. That the rays move in straight, and not in crooked lines, (unless they be re

* Light passes from the Sun to the Earth in 8 minutes and 7 seconds, which is 195,072 miles in one second of time. + A fine net work membrane, in the bottom of the eye.

fracted,) is evident from bodies not being visible if we endeavour to look at them through the bore of a bended pipe; and from their ceasing to be seen by the interposition of other bodies, as the fixed stars, by the interposition of the Moon and planets, and the Sun wholly, or in part, by the interposition of the Moon, Mercury, or Venus.

There is no physical point, (says Melville,) in the visible horizon which does not send rays to every other point; no star in the Heavens which does not send light to every other star. The whole horizon is filled with rays from every point in it; and the whole visible Universe with a sphere of rays from every star. In short, for any thing we know, there are rays of light joining every two physical points in the Universe, and that in contrary directions, except when opaque bodies intervene. A ray of light coming from any of the fixed stars to the human eye, has to pass in every part of the intermediate space between the point from which it has been projected, and our solar system, through rays of light flowing in all directions from every fixed star in the Universe; and in reaching this earth, it has passed across the whole ocean of the solar light, and that which is emitted from the planets, satellites, and comets. Yet in this course, its progress has not been intercepted.

The densities and quantities of light, received upon any given planet, arediminished in the same proportion, as the squares of the distances of that planet from the luminous body are increased; and on the contrary, are increased in the same proportion as these squares are diminished..

When a telescope magnifies the disk of the Moon and planets, they appear more dim than to the bare eye; because the telescope cannot increase the quantity of light in the same proportion that it can magnify the surface, and by spreading the same quantity of light over a given surface, it appears more dim than when beheld with the naked eye.

When a ray of light passes out of one medium into