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GRAVITY, a term used by physical writers to denote the cause by which all bodies move toward each other, unless prevented by some other force or obstacle. The most familiar effect, and that which continually obtrudes itself on our notice, is the weight of bodies, or their tendency toward the centre of the earth. It has not been ascertained, or rendered probable, that gravity is a secondary property of matter; that is to say, that it flows from any of the other known original properties. Sir Isaac Newton, however, was of opinion, that our reasonings on the subject might be simplified, by supposing it to depend on a prodigiously elastic and rare fluid, by him called ether, and assumed to possess an increasing degree of condensation, in parts of space Inore and more remote from the various masses of matter. According to this doctrine, a falling body moves, because it is pressed toward the rarer parts of this extended fluid. We shall leave this theory to its merits, as being neither very perspicuous, nor much related to our subject. Bergman, and others, have considered the chemical and cohesive attractions to be one and the same with the attraction of gravity, but modified in its laws, by variations in the masses, densities, and distances of the particles of bodies. Many difficulties appear at first sight to offer themselves against this supposition. But in truth it cannot be examined at first sight; and requires to be submitted to the rigour of mathematical investigation, which has not yet been done.
The phenomena of particular gravity, or that which respects the earth, or by which bodies descend or tend towards the centre of the earth, are as follow :
1. All circumterrestrial bodies do hereby tend towards a point, which is either accurately, or very nearly, the centre of the magnitude of the terraqueous globe. Not that it is meant that there is any virtue or charm in the point called the centre, by which it attracts bodies; but because this is the result of the gravitation of bodies towards all parts of which the earth consists.
2. In all places equidistant from the centre of the earth, the force of gravity is nearly equal. Indeed, all parts at the earth's surface are not at equal distances from the earth’s centre, because the equatorial parts are higher than the polar
arts by about seventeen miles; as has É. proved by the necessity of making the pendulum shorter in those places, before it will swing seconds. In the new “Petersburg Transactions,” vol. 6 and
cond of time, in the latitude of London; and for other times, either greater or less than that, the spaces descended from rest are directly proportional to the squares of the times, while the falling body is not far from the earth’s surface. 4. This power is the greatest at the earth's surface, from whence it decreases both upwards and downwards; but not both ways in the same proportion ; for, upwards, the force of gravity is less, or decreases as the square of the distance from the centre increases; so that at a double distance from the centre above the surface, the force would be only onefourth of what it is at the surface; but below the surface, the power decreases in such sort, that its intensity is in the direct ratio of the distance from the centre; so that at the distance of halfa semi-diameter from the centre, the force would be but
half what it is at the surface; at one-third
of a semi-diameter the force would be but one-third, and so on. 5. As all bodies gravitate towards the earth, so does the earth gravitate towards all bodies; as well as all bodies towards particular parts of the earth, as hills, &c. which has been proved by the attraction a hill has upon a plumb line, insensibly drawing it aside. Hence the gravitating force of entire bodies consists of those of all their parts; for, by adding or taking away any part of the matter of a body, its gravity is increased or decreased, in the proportion of the quantity of such portions to the whole mass. Hence, also, the gravitating powers of bodies at the same distance from the centre are proportional to the quantities of matter in the bodies. General or universal gravity, is that by which all the planets tend towards one another; and, indeed, by which all bodies *
or particles of matter in the universe tend towards one another. The existence of the same principles of #." in the superior regions of the ieavens as on the earth is one of the great discoveries of Newton, who made the proof of it as easy as that on the earth. This was at first only a conjecture in his mind; he observed, that all bodies near the earth, and in its atmosphere, had the property of tending directly towards it; he soon conjectured, that it probably extended much higher than to any distance to which we could reach to make experiments; and so on, from one distance to another, till he at length saw no reason why it might not extend to the moon, by means of which she might be retained in her orbit, as a stone in a sling is retained by the hand; and if so, he next inferred, why might not a similar principle exist in
the other great bodies in the universe, the
sun, and all the other planets, both primary and secondary, which might all be retained in their orbits, and perform their revolutions by means of the same universal principle of gravitation. He soon realized and verified these by mathematical proofs. Kepler had found out, by contemplating the motions of the planets about the sun, that the area described by a line connecting the sun and planet, as this revolved in its orbit, was always proportional to the time of its description, or that it described equal areas in equal times, in whatever part of its orbit the planet might be, moving always as much the quicker as its distance from the sun was less. And it is also found, that the satellites, or secondary planets, respect the same law in revolving about their primaries. But it was soon proved, by Newton, that all bodies moving in any curve line described on a plane, and which, by radii drawn to any certain point, describes areas about the point proportional to the times, are impelled or acted on by some power tending towards that point. Consequently, the powerby which all these planets revolve, and are retained in their orbits, is directed to the centre about which they move, viz. the primary planets to the sun, and the satellites to their several primaries. Again, Newton demonstrates, that if several bodies revolve with an equal m2tion in several circles about the same centre, and that if the squares of their periodical times be in the same proportion as the cubes of their distances from the common centre, then the centripetal forces
of the revolving bodies, by which they tend to their central body, will be in the reciprocal or inverse ratio of the squares of the distances. But it had been agreed on by the astronomers, and particularly Kepier, that both these cases obtain in all the planets; and therefore he inferred, that the centripetal forces of all the planets were reciprocally proportional to squares of the distances from the centres of their orbits. Upon the whole, it appears that the planets are retained in their orbits by some power which is continually acting upon them : that this power is directed towards the centre of their orbits: that the intensity or efficacy of this power increases upon an approach towards the centre, and diminishes on receding from the same, and that in the reciprocal duplicate ratio of the distances; and that by comparing this centripetal force with the force of gravity on the earth, they are found to be perfectly alike, as may easily be shown in various instances. For example, in the case of the moon, the nearest of all the planets, the rectilinear spaces described in any given time, by a body urged by any power, reckoning from the beginning of its descent, are porportionate to those powers. Consequently, the centripetal force of the moon, revolving in its orbit, will be to the force of gravity on the surface of the earth as the space which the moon would describe in falling, during any small time, by her centripetal force towards the earth, if she had no motion at all, to the space a body near the earth would describe in falling by its gravity towards the same. Now, by an easy calculation of these two spaces, it appears that the former force is to the latter as the square of the semi-diameter of the earth is to the square of that of the moon's orbit. The moon’s centripetal force, therefore, is equal to the force of gravity; and consequently these forces are not different, but they are one and the same ; for if they were different bodies, acted on by the two powers conjointly, they would fall towards the earth with a velocity double to that arising from the sole power of gravity. It is evident, therefore, that the moon’s centripetal force, by which she is retained in her orbit, and prevented from running off in tangents, is the very power of gravity of the earth extended thither. See “Newton’s Principia,” lib. i. prop. 45, cor. 2. and lib. iii. prop. 3; where the numeral calculation may be seen at full length.
The moon, therefore, gravitates towards the earth, and reciprocally the earth towards the moon, and this is also farther confirmed by the phenomena of the tides. The like reasoning may also be applied to the other planets. For as the revolutions of the primary planets round the sun, and those of the satellites of Jupiter and Saturn round their primaries, are phenomena of the same kind with the revolution of the moon about the earth ; and as the centripetal powers of the primary are directed towards the centre of the sun, and those of the satellites towards the centres of their primaries; and, lastly, as all these powers are reciprocally as the squares of the distances from the centres, it may safely be concluded that the power and causes are the same in all. Therefore, as the moon gravitates towards the earth, and the earth towards the moon, so do all the secondaries to their primaries, and these to the secondaries; and so also do the primaries to the sun, and the sun to the primaries. Newton’s Princip. lib. iii, prop. 4, 5, 6; Greg. Astron. lib. i. sect. 7, prop. 46 and 47. The laws of universal gravity are the same as those of bodies gravitating towards the earth, before laid down. See Asthonoxiy, ATTRAction, Grog RAPHY. GRAvity, specific. Boyle is among the first of our philosophers, who suggested the advantage that chemistry and mineralogy might derive from an attention to the specific gravities of bodies. Much advantage may indeed be derived from this property in the general determination of the classes of minerals, and the purity of some metallic bodies; and it is very probable, that an attention to the specific gravities, capacities for heat, fusibilities, volatilities, laws of crystallization, elasticity, hardness, tenacity, malleability, and some other obvious specific roperties of bodies, may produce a more intimate acquaintance with the mutual actions of their particles, than any we have hitherto aqquired. Annexed to this article is a table of specific gravities, from various authors. It appeared useless to carry it to more than four places of figures, as the temperatures are not noted; and the various specimens of the same substance often differ in the third figure. Besides this, it is remarked
by Nicholson, in his “Chemical Dictionary,” that the fifth figure changes in waterat every three degrees of Fahrenheit's thermometer; that lead, tin, and probably all other metals, though cast out of the same fusion, will vary in their specific gravities in the third figure, from circumstances not yet determined, but most likely from the cooling, as is seen in the hardening of steel; that salts, and other artificial preparations, retain more or less of the solvent they were separated from, according to the temperature at which the crystallization was effected; and that all parts of organized substances not only differ, according to the place of their production, their age, and other circumstances, but likewise from their dryness, moisture, and manner of preservation.
The specific gravity of solids is determined by weighing them, first in air, and then in water. The loss of weight, arising from the action of the water, is equal to that of a mass of the fluid possessin the same dimensions as the solid itself. Whence it is easy to construct a general table of specific gravities, by reducing the proportion of the absolute weight to the loss sustained by immersion, into terms of which that expressing water shall be unity. If the solid be so light as to float upon water, it is convenient to attach to it a heavier body sufficient to cause it to sink, but the weight of which in water must be added in computing the loss. The specific gravity of fluids is ascertained by weighing a known body immersed in them. For the loss by immersion will accurately show the weight of the same bulk of the fluid; and, consequently, the proportion of these several quantities to the loss the same solid sustained in water being reduced, as in the other case, to the common standard of unity, will exhibit the specific gravity. Other methods are likewise used in experiments with fluids. Thus equal bulks of different fluids may be weighed by filling a small bottle with a ground stopper with each respectively, and from their several weights the weight of the bottle and stopper must be deducted. Or, otherwise, the instrument called the hydrometer may be used. See HyphoMeten. This possesses the advantage of portability, speed, and a degree of accuracy, not easily obtained by the use of ordinary balances.
Shewing the Specific Gravity of Metals and other bodies to Rain Water, and the weight of a Cubic Inch of each in parts of a Pound Avoirdupoise. The Number in the Column, Specific Gravity, shows the Ounces Avoirdupoise in the Cubic Foot
of each body.
For the Specific Gravities of different kinds of elastic fluids, see the Table at the
end of the article GAs.
Gnavity, in music, is the modification of any sound, by which it becomes deep or low in respect of some other sound. The gravity of sounds depends on the thickness and distension of the chords, or the length and diameter of the pipes, and in general on the mass, extent, and tension of the sonorous bodies. The larger and more lax the bodies, the slower will be the vibrations and the graver the sounds.
GREASE. See PARItiery.
GREAT circle sailing, the manner of conducting a ship in, or rather pretty near the arch of a great circle, that passes through the zenith of the two places, viz. from whence she came, and to which she is bound.
GREEK church. In the eighth century there arose a difference between the eastern and western churches, which, in the course of about two centuries and a half, ended in a total separation. The Greek, or Eastern, or, as it is sometimes called, the Russian Church, spread itself over the eastern parts of Europe. It bears a considerable resemblance to the church of Rome, but denies the infallibility and supremacy of the Pope : it rejects also the worship of images, and the doctrine of consubstantiation, or the union of the body of Christ with the sacramental elements. The administration of baptism is performed by immersing the whole body. The Greek church has the same division of clergy, and the same distinction of ranks and offices, with the church of Rome.
GREEN, one of the original colours excited by the rays of light. See ChnoMArics, CoLochs, and Optics. The green colour of plants has been shown, by the French chemists, to depend upon the absorption of carbonic acid, and it is supposed that the leaves of plants have the power of decomposing the carbonic acid, and water also ; the oxygen they emit, while the carbon and hydrogen enter into the composition of the inflammable parts of the plant.
GREEN Brunswick, a pigment used by German artists, and known in our shops under that lame. It is made by saturating cold water with muriated ammonia, and dding three times as much copper clipping as ammonia. The moisture is to be evaporated, taking care that no dust be allowed to get to it. The muriate of ammonia is decomposed by the copper, which is itself corroded and converted into a green oxide. It is then to be digesed in successive portions of alcohol, as long as any green oxide is taken up; the
solutions are now to be added together, and the liquor to be driven off by a moderate heat; the residue is the pigment required. GREEN cloth, a board or court of justice, held in the counting-house of the king's household, composed of the lordsteward, and officers under him, who sit daily. To this court is committed the charge and over-sight of the king's household in matters of justice and government, with a power to correct all offenders, and to maintain the peace of the verge, or jurisdiction of the court-royal; which is every way about two hundred yards from o last gate of the palace where his Majesty resides. It takes its name, board of green-cloth, from a green cloth spread over the board where they sit. Without a warrant first obtained from this court, none of the king's servants can be arrested for debt. GREEN finch. See FRINGilla. GREENHOUSE, or conservatory, a house in a garden contrived for sheltering and preserving the most tender and curious exotic plants, which, in our climate, will not bear to be exposed to the open air during the winter season. These are generally large and beautiful structures, equally ornamental and useful. GREGORIAN calendar, that which shows the new full moon, with the time of Easter,and the moveable feasts depending thereon, by means of epacts, disposed through the several months of the Gregorian year. o GREgon IAN epoch, the epocha, or time whence the Gregorian calendar or computation took place. The year 1808 is the 226th year of that epocha. GREgon IAN year, the Julian year corrected, or modelled in such a manner, as that three secular years, which in the Julian account are bissextile, are here common years, and only every fourth secular year is made a bissextile year. The Julian computation is more than the solar year by eleven minutes, whichin one hundred and thirty-one years amounts to a whole day. By this calculation, the vernal equinox was anticipated ten days from the time of the general council of Nice, held in the year 325 of the Christian aera, to the time of Pope Gregory XIII. who therefore caused ten days to be taken out of the month of October, in 1582, to make the equinox fall on the twenty-first of March, as it did at the time of that council; and to prevent the like variation for the future, he ordered that three days should be abat