GRAVITA, an Italian term used in music, signifying that it is to be performed with an earnest and dignified expression, while the movement progresses in a slow, marked, and solemn time.
All bodies, when raised into the air, and left unsup ported, fall to the earth in lines perpendicular to it. The force which causes them to do so. is termed gravity, and, universal experience shows, acts towards the earth's center; more strictly, it acts perpendicularly to the surface of still water. But if a body, as a stone, be projected obliquely into the air, it is made to describe a curved path, having a highest point, vertex, or apogee; and when it meets the earth in its descent, its direction is not towards the center, but inclined to it at the angle of projection. See PROJECTILES. Observing this, and that the body, if not interrupted by the earth's surface, would continue to move in a curve, with its tangent always away from the center, it is easy to imagine that if not interrupted, it might circulate round the center as the moon does round the earth. Next, knowing that the force of gravity is exerted at all accessible heights above the earth, the question arises—May it not be exerted as far off as the moon? which we know to influenced by same force which continually deflects her from the tangent to her orbit, and makes her circulate round the earth. See CENTRAL FORCES. Observing now the time of revolution of the moon, and calculating its centrifugal force, which we know must equal the centripetal force, we put the question: Is this force the same as gravity? The answer is, that it is a force 3,600 times less energetic. If, then, gravity be the force which really holds the moon-. to her path, it must be explained why it acts upon her so much more feebly than it would, were she a body on the earth's surface. The explanation is given at once if we snppose gravity to be a force whose energy diminishes with increase of distance, and if inversely as the squares of the distances at which it is exerted, for the distance of the moon from the earth's center is just about 60 times that Of the earth's surface from its center, and 3,600: 1 • : : 1. We infer that it does so from the fact, that there is nothing inadmissible in such a diminution of energy with increase of distance—that, on the contrary, there are many analogies for it, as in the emanations of light and heat; and in the argument drawn from the necessity of otherwise supposing some other force than gravity to be employed in deflecting the moon, and the force of gravity to cease at some unknown level. On these views, and a generalization to be afterwards mentioned, Newton is understood to have at first rested his law of universal gravitation: " Every particle of matter in the universe attracts every other particle with a force directly proportioned to the mass of the attracting particle, and inversely to the square of the distance between them "—a law, the truth of which, since it was first broached, has been put beyond all question by the most complete body of predictions, fulfilled to the letter, that can be cited in support of any law of nature.
Before, however, the argument on the extension of terrestrial gravity to the sphere of the moon could have become pregnant with so great a result, much investigation had to take place in other fields; and, in fact, Newton had, previously to conceiving the• law, explained the three great Keplerian laws of order obtaining in the solar system by reference to an attractive force residing in the sun. These laws are-1. That the plan ets revolve round the sun in ellipses, having the sun for a common focus: 2. That every planet moves in such a way that the line drawn from it to the sun sweeps over equal areas in equal times: 3. That the squares of the times occupied by the several planets in their revolutions in their elliptic orbits, are proportional to the cubes of their mean dis tances from their common focus, the sun. From the law of equal areas, Newton inferred that every planet is retained in its orbit by a force of attraction directed towards the center of the sun; from the orbits being elliptical, he inferred that in each case this force varies in intensity according to the inverse square of the body's distance from the sun; while from the third law he inferred the homogeneity of the central force through out the solar system. It was then, after being familiar with the notion of terrestrial gravity, and its action, through the researches of Galileo, Huyghens, and Hooke, and with the notion of a central force acting inversely as the square of the distance of its object, through his explanations of the laws of Kepler, that he put to himself the ques tion: Is not the force with which the moon gravitates to the earth the same with gravity ?— the force which causes a stone to fall on its surface. A question answered affirmatively on the supposition of gravity, like the sun's attraction, being a force diminishing with increase of distance, and according to the same law. The result was to bring the whole solar system, the planets and the sun, and satellites and their planets—the satellites being observed to obey the same laws of order with reference tt their primaries that the latter obeyed in reference to the sun—under the law of gravitation. And the imagi nation lifted up by the grandeur of the conception, would refuse to limit the operation of that law to our own system, were there no facts to entitle us to extend it beyond The phenomena of double stars, however, of themselves justify the extension and the statement of the law as we have given it in universal terms. It:may be observed, in conclusion, that the Keplerian laws, which may be said to have been the basis of New ton's researches, are, owing to perturbations caused by the mutual action of the planets, etc., only approximately correct; and that these perturbations afford, when examined, a further proof of the truth and universality of the law of gravitation.
For a notice of speculations as to the nature of the law of gravitation, see FORCE: See also FALLE1D BODIES, PROJECTILES, etc.