with more life and vigour. See ENGRAV

ING.

GRAVIMETER, the name given by M. Guyton to an instrument for measuring specific gravities: he adopts this name rather than either arcometer or hydrometer, because these latter terms are grounded upon the supposition that a fluid is always the thing weighed; whereas, with regard to solids, the liquid is the known term of comparison to which the unknown weight is referred. Guyton's gravimeter is executed in glass, and is of a cylindric form, being that which requires the smallest quantity of fluid, and is on that account preferable, except so far as it is necessary to deviate for the security of a vertical position. It carries two basins, one of them superior, at the extremity of a thin stem, towards the middle of which the fixed point of immersion is marked. The other, or lower basin, terminates in a point; it contains the balls, and is attached to the cylinder by two branches. The moveable suspension, by means of a hook, has the inconvenience of shortening the lever which is to secure the vertical position. The cylinder is three fourths of an inch in diameter, and 6.85 inches in length. It carries in the upper basin an additional constant weight of five grammes, or one hundred and fifteen grains. These dimensions might be increased so as to render it capable of receiving a much more considerable weight; but this is unnecessary. M. Guyton has added a piece which he calls the plonguer, because, in fact, it is placed in the lower basin when used, and is consequently entirely immersed in the fluid. It is a bulb of glass loaded with a sufficient quantity of mercury, in order that its total weight may be equal to the constant additional weight added to the weight of the volume of water displaced by this piece. It will be readily understood that the weight being determined at the same temperature at which the instrument was originally adjusted, it will sink to the same mark on the stem, whether it is loaded with a constant additional weight in the upper basin, or whether the effect of this weight be produced by the additional piece in the lower dish. From this explanation there will be no difficulty in seeing how this instrument may be adapted to every case in practice.. It may be used, 1. For solids. The only condition will be, that the absolute weight of the body to be examined shall be rather less than the constant additional weight, which in this instrument is about 115 grains. 2. For

liquids of less specific gravity than water, the instrument, without the additional weight above-mentioned, weighs about four hundred and fifty-nine grains, in the dimensions before laid down. It would be easy to limit its weight to the utmost accuracy. We have therefore the range of one-fifth of buoyancy, and consequently the means of ascertaining all the intermediate densities from water to the most highly rectified spirit of wine, which is known to bear in this respect the ratio of eight to ten with regard to water. 3. When liquids of greater specific gravity than water are to be tried, the constant weight being applied below by means of the additional piece, which weighs about one hundred and thirty-eight grains, the instrument can receive in the upper basin more than four times the usual additional weight, without losing the equilibrium of its vertical position. In this state it is capable of shewing the specific gravity of the most concentrated acids. 4. It possesses another property, namely, that it may be used as a balance to determine the absolute weight of such bodies as do not exceed its additional load. 5. Lastly, the purity of the water being known, it will indicate the degrees of rarefaction and condensation in proportion to its own bulk. To find the specific gravity of any solid by the gravimeter, observe this rule: "From the weight in the upper dish, when the instrument is properly immersed in the unknown fluid, take the weight which is placed with the body in the same scale at the like adjustment. The remainder is the absolute weight of the solid. Multiply this by the specific gravity of the fluid, and reserve the product. From the additional weight, when the body is placed in the lower basin, take the weight when it was placed in the upper. The remainder will be the loss of weight by immersion. Divide the reserved product by the loss by immersion, and the quotient will be the specific gravity of the solid with regard to distilled water at the standard temperature and pressure." To find the specific gravity of a fluid, proceed thus: "To the weight of the gravimeter add the weight required in the upper basin to sink it in the unknown fluid." Again, "To the weight of the gravimeter add the weight required in the same manner to sink it in distilled water. Divide the first sum by the latter, and the quotient will be the specific gravity of the fluid in question." See SPECIFIC GRAVITY, HYDROSTATICS, and Hy

DROMETER.

GRAVING. See ENGRAVING. In sea affairs the word graving is used for the act of cleaning a ship's bottom, when she is laid aground during the recess of the tide. See BREAMING and CAREENING.

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 more 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 follows:

1. All circumterrestrial bodies do here by 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 VOL. III.

are higher than the polar parts by about seventeen miles; as has been 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 7. M. Krafft gives a formula for the proportion of gravity in dif ferent latitudes on the earth's surface, which is this:

y=(1+0.0052848 sine 2x) g;

where g denotes the gravity at the equator, and y the gravity under the other latitude a.

3. Gravity equally affects all bodies, without regard either to their bulk, figure, or matter: so that abstracting from the resis tance of the medium, the most compact and the most loose, the greatest and the smallest bodies would all descend through an equal space in the same time, as appears from the quick descent of every light body in an exhausted receiver. The space which bodies do actually fall in vacuo, is 16, feet in the first second 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 one-fourth 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 half a 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 gra vity is increased or decreased, in the proСс

portion 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 gravitation in the superior regions of the heavens 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; aud 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 decribed 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 bo. dies 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 power by 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 equable motion 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 Kepler, 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 proportionate 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 semidiameter 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, 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