Gravitation

[Fr.] ( Physics.) A term often used synonymously with gravity, to denote that mutual tendency which all bodies in nature have to approach each other, with forces which are directly as their ma es, and inversely proportional to the squares of their distances. That every particle of matter in the universe has a disposition to pre towards, and, if not opposed, to approach to every other, is a fact of which we derive the knowledge partly from our constant experience of what takes place at the earth's surface, and partly by reasoning from the observed motions of the celestial bodies. This mutual tendency of all the particles of matter to each other is called the attraction of gravitation. In reference to any particular body, or ma of matter, the aggregate attraction of all its particles is usually called simply its gravity. Universal experience demonstrates that all heavy bodies, when unsupported, fall toward the surface of the earth. The direction of their motion may be ascertained by a plumb-line; and it is found to be always perpendicular to the level surface of the earth - that is, to the surface of stagnant water. But the earth is very nearly spherical, and a line perpendicular to the surface of a sphere must pa through its centre; hence the direction of a body moving in consequence of the force of terrestrial gravity is towards the centre of the earth. And this is the direction in which it must move if the force of gravity is the resultant of the attraction of all the particles of terrestrial matter on the falling body; for it has been demonstrated by Newton that a sphere attracts an exterior body in the same manner as if all its matter were condensed into a single point at its centre. As bodies when left without support fall from all heights to which they may be carried, it may be inferred that gravity acts on them during the whole time of their descent, and is therefore a uniformly accelerating force. This might also be inferred from the fact, which is easily rendered tenable, that bodies which fall from a greater height arrive at the earth with a greater velocity. But Galileo was the first who proved, by experiments, that the acceleration of falling bodies is uniform, and that the spaces through which they descend are consequently as the squares of the time of descent. The best method of showing, experimentally, that gravity is a uniformly accelerating force is by means of Attwood's machine, the description and figure of which are given under the word ATTWOOD. Terrestrial gravity acts equally on all bodies, that is to say, impre es on all of them an equal quantity of motion, whatever their nature may be. This property of gravity was also demonstrated by Galileo. In different hollow spheres, of equal weight and diameter, he enclosed equal weights of different substances: the spheres were suspended by strings of equal length, and made to vibrate in very small arcs, when it was found that the time of oscillation was the same in all of them. Common experience would seem to be at variance with this result. Light bodies, as feathers, paper, ., fall slowly and irregularly; and some substances, as smoke, vapors, ., even ascend. But this, as is well known, arises from the buoyancy of the atmosphere. In the exhausted receiver of an air-pump a piece of gold 1 1 IMG:content-0426.png:[ocr errors] and a feather fall with the same speed, and strike the bottom at the same time. - Having ascertained the law according to which gravity acts on bodies at the surface of the earth, the next question is to determine its absolute intensity, or the velocity which it communicates to a body falling freely in a given time. On account of the rapidity of the descent of heavy bodies, this cannot be done by direct experiment; nor could Attwood's machine be employed for the purpose with sufficient certainty. The only mode by which an accurate result can be obtained is by measuring the length of a pendulum which makes a given number of oscillations in a given time. From experiments made with the greatest care, it appears that the extreme amount of the variation of the gravitating force between the equator and the poles is one part in 194 of the whole quantity; that is to say, any body which at the equator weighs 194 lbs., if transported to the pole would weigh 195 lbs. The difference of gravitation, therefore, at the equator and the poles, is expre ed by the fraction Tor. Now it has been demonstrated by Newton that the ratio of the centrifugal force at the equator to gravitation there is This is considerably smaller than the fraction T the difference, which is, arises from the oblate figure of the earth, in consequence of which a body placed at the pole is at a le distance from the centre than one at the equator, and is therefore attracted more than it would be at the equator, even if the earth stood still, and there be consequently no centrifugal force. From this it may be readily understood that the variation of the intensity of gravity, or, in other words, the figure of the earth, may be deduced from the number of oscillations of the pendulums of the same construction would perform in 24 hours in places situated under different latitudes; or it might also be determined from a comparison of the different lengths which must be given to a pendulum in order that it may perform in every place the same number of oscillations in a given time. -Universal Gravitation. Although Kepler made some shrewd gue es at the motions of the planets and the causes of tides and other similar phenomena, it is to Sir Isaac Newton that we are indebted for the principles and applications of universal gravitation , and through it, by means of pure geometry, we are able to po e the correct information that we have with regard to the movements of the earth, sun, and moon, and other b avenly bodies. The first rule is one which is very comprehensive, and which gives a good idea of the whole basis of the science; it is, that the attraction of one body upon another body does not depend upon the ma of the body which is attracted, but is the same whatever be the ma of the body so attracted, provided that the distances be the same. For instance, the planet Jupiter attracts the sun, and also attracts the earth; but although the sun's ma is 300,000 times that of the earth, yet the attraction of Jupiter on the earth is exactly equal to his attraction of the sun, because the earth and sun are equally distant from Jupiter. One of the simplest illustrations of this force is that of throwing a stone in a straight or horizontal plane, when the stone's course will be evolved in a curve, and the stone at length will drop to the ground. The flights of shot and shell are likewise illustrative of the same rule. (See GUNNERY.) Newton, before applying his theory of universal gravitation, sought for a law by which he could regulate the diminishing intensity of the same, and, after several experiments and calculations, he laid down the rule that the force of gravity diminishes exactly as the square of the distance increases, or, in other words, that the attractive force of the earth at the distance of the moon must be as much le than it is at the surface of the earth, as the square of the radius of the earth is le than the square of the moon's distance from the earth. Newton also found, that since the true diameter of the moon is to the true diameter of the earth as 100 is to 365, the ma of matter in the moon is to the ma of matter in the earth in the proportion of 1 to 39,788; and also that the accelerative gravity on the surface of the moon is to the accelerative gravity on the surface of the earth, as 1 is to 3, or is just of that of the earth. He also proved that bodies moving under an attractive force which diminishes according to the inverse square of the distance, must describe conic sections, having a focus at the centre of force; and also that they must conform to the laws of motion which Kepler discovered to belong to the planetary orbs. Newton likewise was succe ful in determining that most of the inequalities of the moon and the planets are consequences of the mutual gravitation of the different bodies which compose the various systems upon each other: and in addition, that the same incomprehensible power not only regulates the motions of the different planets and satellites, but also causes the prece ion of the equinoxes, produces the tidal action, and determines the figure of the earth. Gravitation, as applied to the celestial bodies, when we consider its effects, enables us to form many conclusions as to its nature, mode of action, and influence. We see that gravity is a force which is transmitted from body to body instantaneously , and not succe ively; for were we able to measure its transmi ion, that is, if we consider it in the light of being transmitted succe ively, we would find that the secular variation of the mean lunar motion would be sensibly affected. If we consider the question whether gravity is affected by the density of the bodies through which it has to pa in order to attract other bodies, we would be forced to agree with Laplace, that it is of so subtile and all powerful a matter, or force, that not even the densest bodies in the universe can offer any obstacle to its free pa age, or retard its effects on the body to be acted upon. In concluding this subject, it may be said 3 IMG:content-0427.jpg:[graphic][subsumed][subsumed] that if the earth's flattening at each of its poles were greater or le than 1-300th of its diameter, then the effect of this alteration on the moon would, in changing the position of its fundamental plane, thus produce an inequality in the longitude greater or le than 8", by which the moon is sometimes before or behind her mean place. And, consequently, the deduction can be drawn, that by observing the moon, the oblatene of the earth can be discovered. As this theory has been found to be true and just in its foundation, it is one of the most striking testimonies of the correctne of Newton's laws of universal gravitation.