superficies of the earth, is very heavy, if it could be carried to a distance as high as the Moon, would be very light.

And, finally, the proportion of the increase or decrease of this gravity in bodies approaching to, or receding from, each other, is such, that its force is reciprocal in a duplicate proportion, or as the squares of their distances: for example, a body which, at the distance of ten diameters of the earth, weighs a hundred pounds, would, if its distance were but half as far, weigh four times as much; and if but a third part as far, nine times. as much. In like manner, the force which, upon the superficies of the earth, would support a hundred pound weight, if it were twice as far off the centre, could support four times the weight, and, if at three times as far off, could support nine times the weight.

It must be observed, moreover, that the motion. of falling bodies is increased, in successive intervals, in the proportion of odd numbers, 1,3,5,7,9, &c. so that the spaces they run through are as the squares of the times; that is, if a heavy body descended one foot in a second of time, it would descend four feet in two seconds, nine feet in three seconds, &c. Or correctly, at least in the latitude of London, a body falling freely, passes through 16 feet in the first second; four times 16, or 64 feet, in the first two seconds; nine times 167, or 144 feet in the first three seconds; and so on.

Such are the principal properties of gravity, from the observation of which our immortal Newton deduced his sublime theory of the universe. It may be pleasing to the reader, to know the very simple incident, which, leading to the most extensive and complicated calculations, was produc tive of these noble and wonderful discoveries... It

is thus related, on the authority of his friend and commentator, Dr. Pemberton :

Happening one day, in the year 1666, when only twenty-five years of age, to be sitting under an apple tree, an apple falling upon his head, suggested a variety of reflections. The phenomena of falling bodies in particular engaged his attention; and, extending his researches to the heavens, he began to investigate the nature of motion in general. Because there is motion, he reasoned, there must be a force that produces it. But what is this force? That a body, when left to itself, will fall to the ground, is known to the most ignorant; but if you ask them the reason of its thus falling, they will think you either a fool or a madman. The circumstance is too common to excite their wonder, although it is so embarrassing to philosophers, that they think it almost inexplicable.

It is impossible to follow this great philosopher through all his reasonings; but we may recapitulate some of the principal phenomena of this simple, yet wonderful, this obvious, yet mysterious law of nature. If I take a stone, and let it drop from my hand, it falls immediately to the ground, and would fall still farther, if there was a hole in the earth, and nothing to prevent, its passage. In like manner, when a stone, or any other substance, is placed upon a table, it presses the table with the same force by which it would fall to the ground, if left to itself; and when a body is suspended at the end of a string, the force that impels it downward, stretches the string, and, if it be not sufficiently strong, will break it.

The cause of this falling propensity in all bodies, whatever it be, is called Gravity, Gravitation, or Attraction; and when a substance is said to be heavy, nothing more is meant, than a reference to the tendency which it has to fall to the ground,

or to the force with which it presses on any substance that supports it. The weight and gravity of a body, therefore, may be taken for the same thing in different senses, for both refer to the force by which the body is impelled toward the earth; gravity being the tendency of bodies towards the centre of the earth; weight, the force which is necessary to counteract that tendency, or to prevent its producing motion downwards.

The antients were very imperfectly acquainted with this obvious property of bodies. They believed that there were substances, such as vapours and smoke, which from their light nature would ascend. There is no foundation, however, for opinion, nor for that of absolute levity in general; for, in a space void of air, all bodies whatever, smoke or a stone, a feather or a guinea, fall to the bottom in the same time. The distinction, therefore, between light and heavy bodies is merely relative; as they have all a similar propensity to fall to the earth.

The same body, conveyed to different places upon the surface of the earth, will not always haye the same weight: for the weight of a body under the equator (as I demonstrated in my last paper) is less than at either of the poles; and in every other situation, it varies in a certain proportion to the latitude of the place. For the centrifugal force produced by the rotatory motion of the earth round its axis is opposed to the force of gravity, is greatest at the equator, least at the poles, and at intermediate points varies as the square of the cosine of the latitude: the gravity of a body, therefore, will be least at the equator, it will continually increase with the latitude of the place, and will be greatest at the poles.

The earth is nearly a globe, and gravity acts

perpetually in straight lines, which are perpendicular to its surface. Suppose a hole could be bored through it, and that a body were placed at the centre, what would be its weight? Evidently nothing: it would remain in that situation, unsuspended and unsupported. The force of gravity being constantly directed toward the centre, it could have no propensity to move either way, and would, therefore, have no weight. For the same reason, if a body were thrown into this hole, from the surface of the earth, the velocity acquired by the repeated impulses of gravity during the time of its fall, would carry it to the opposite extremity of the perforation, whence it would return, and continue to move backward and forward for ever, if the medium had no resistance. These two cases may be illustrated by the pendulum of a clock, which, if once stopped, would move no more, till put again into motion by some impelling force, when (supposing the constant action of the weight) it would continue its vibrations.

In applying his reflections on the nature of falling bodies to the celestial motions, Newton soon perceived, that the force of gravity was not confined to the surface of our globe; it being found to act alike at the bottom of the lowest vallies, and at the summit of the most lofty mountains. This led him to conjecture, that it might extend as far as the Moon, and be the means of retaining her in her orbit. Imagine the Moon, he rea soned, at the first moment of its creation, to have been projected forward, with a certain velocity, in a rectilinear direction; then, as soon as it began to move, gravity would act upon it, and impel it toward the centre of the earth. But as a body, impelled by two forces, will follow the direction of neither, the Moon, so circumstanced, would nei

ther proceed directly forward, nor fall directly. downward, but keep a middle course, and move round the earth in a curvilinear orbit.

This may be more fully illustrated, by attending to the motion of a shot, or any other projectile. A ball shot from the mouth of a cannon, in a horizontal direction, does not fall to the ground till it has proceeded to a considerable distance; and if it be discharged from the top of a high mountain, it will fly still farther before it comes to the earth. Increase the force and the height, and the distance will be augmented accordingly. And thus, in imagination at least, we can suppose the ball to be discharged with such velocity, that it will never come to the ground, but return to the place whence it set out, and circulate continually round the earth, in the manner of a little moon.

Thus proceeding in his reflections, Newton discovered the admirable provision of the great Creator to prevent the evagation of the planets, and to retain them exactly within the bounds of their orbits. This he has demonstrated to be effected by gravity, and that gravity and motion completely solve all the phenomena of the planetary revolutions, both primary and secondary. By establishing this one principle in philosophy he has fully explained the system of the world, so far as it relates to this globe, and to all the rest of the planets that regard the Sun as their centre.

Such is the Newtonian system of universal gravitation or attraction. But what is this principle, which gives life and motion to inanimate beings, and how does it act? The effects are visible, but the agent that produces them is hidden from our senses. It eluded the search of Newton himself; he that soared to the utmost regions of.. space, and looked through nature with the eve