Gravity, in Physics, the natural tendency of bodies towards a centre. Terrestrial or particular gravity is that by which bodies descend or tend towards the centre of the earth; the phenomena of which are as follows: - 1. All circumterrestrial bodies tend towards a point which is either accurately or nearly the centre of magnitude of the terraqueous globe. 2. In all places equidistant from the centre of the earth the force of gravity, caeteris paribus, is equal. The force of gravity is not equal on all parts of the earth's surface for two reasons: first, because, as the earth is not a sphere, but a spheroid, all parts of its surface are not equidistant from its centre; and secondly, the gravity is different in different latitudes, by reason of variations in the centrifugal force, occasioned by the earth's rotation, the increment of gravity on this account being as the square of the cosine of the latitude. 3. Gravity affects equally all bodies, without regard either to their bulk, figure, or matter; so that in a perfectly unresisting medium, the most compact and the loosest, the greatest and the smallest bodies would descend through an equal space in the same time.

The space through which bodies do actually fall in vacuo is 161/2 feet in the first second of time in the latitude of London, and for other portions of time either greater or less the spaces are as the squares of the times. 4. Gravity is greatest at the earth's surface, from whence it decreases both upwards and downwards, but not at the same ratio in each direction; the diminution of the force upwards being as the square of the distance from the earth's centre; whilst downwards, the decrease is in the direct ratio of the distance from the centre.

General, or Universal Gravity, is that in consequence of which all the planets tend to one another; and indeed all the bodies and particles of matter in the universe tend to one another.

Gravity, specific, is the relative gravity of any body or substance, considered with regard to some other body which is assumed as a standard of comparison, and this standard by universal consent and practice is rain water or distilled water, and by a very fortunate coincidence, at least to English philosophers, it happens that a cubic foot of water weighs 1000 ounces avoirdupoise, and consequently assuming this as the specific gravity of rain water, and comparing all other bodies with this, the same numbers that express the specific gravity of bodies will at the same time denote the weight of a cubic foot of such bodies in avoirdupoise ounces. From the preceding definition may be drawn the following laws of the specific gravity of bodies. l. In bodies of equal magnitudes, the specific gravities are directly as the densities, or as their weights. 2. In bodies of the same specific gravities, the weights will be as the magnitudes. 3. In bodies of equal weights, the specific gravities are inversely as the magnitudes. 4.

The weights of different bodies are to each other in a compound ratio of their magnitudes and specific gravities. 5.

When a body is specifically heavier than a fluid it loses as much of its weight when immersed in it as is equal to the weight of a quantity of the same fluid of equal bulk. 6. If the gravity of the fluid be greater than that of the body, then the weight of the quantity of fluid displaced by the part immerged, is equal to the weight of the whole body. The specific gravity of solid bodies is usually determined experimentally by means of the "Hydrostatic Balance," (See Balance;) but for ascertaining the specific gravity of liquids, an instrument termed a "Hydrometer" is usually employed. A description of a variety of these instruments will be found under the head Hydrometer.

The late Professor Leslie invented a new and singularly simple and ingenious method for ascertaining the specific gravity of solids. All substances of this class are more or less porous; and the pores being filled with air which is not expelled when the substance is immersed in water causes their specific gravity when ascertained by the hydro3tratic balance to appear less than it really is. In Mr. Leslie's method this source of error is avoided, and some of the results obtained in consequence are extremely curious. The instrument employed consists of a glass tube ac, about three feet long, and open at both ends; the wide part a b is about four-tenths of an inch in diameter; the part b c about two-tenths. The two parts communicate at b by an extremely fine slit, which suffers air to pass, but retains sand or powder. The mouth at a is ground smooth, and can be shut so as to be air-tight by a small glass plate f. The substance whose specific gravity we wish to find is first reduced to powder, which is then put into the wide part of the tube a b, which may either be filled or not. The tube being then held in a vertical position has the narrow part immersed in mercury, contained in an open vessel x, till the metal rises within to the gorge b.

The lid is then fitted on air-tight at a.

In this state it is evident there is no air in the tube, except that mixed with the powder in the cavity a b. Suppose the barometer at the time to stand at 30 inches, and that the tube is lifted perpendicularly upwards, till the mercury stands in the inside of b c, at a point e 15 inches, (or one half of 30,) above its surface, in the open vessel; it is evident, then, that the air in the inside of the tube is subjected to a pressure of exactly half an atmosphere: and of course it dilates and fills precisely twice the space ft originally occupied. It follows, too, that since the air is dilated to twice its bulk the cavity a b contains just half what it did at first; and the cavity b e now containing the other half, the quantity of air in each of these parts of the tube is equal. In other words, the quantity of air in b e is exactly equal to what is mixed with the powder in a b, and occupies precisely the same space which the whole occupied before its dilatation. Let us now suppose the powder to be taken out, and the same experiment repeated, but with this difference, that the cavity a b is filled with air only.

It is obvious that the quantity being greater it will, when dilated to double the bulk under a pressure of fifteen inches, occupy a larger space, and the mercury will rise, let us suppose, only to d. But the attenuated air in the narrow tube always occupies exactly the space which the whole occupied at ordinary atmospheric pressure; and this space is therefore, in the one case, the cavity b e, and in the other b d. Hence it follows that the cavity e d, which is the difference between these, is equal to the bulk of the solid matter in the sand. Now by marking the number of grains of water held by the narrow tube b e on a graduated scale attached to it, we can find at once what is the weight of a quantity of water, equal in bulk to the solid matter in the sand; and by comparing this with the weight of the sand, we have its true specific gravity. Aware that some solid bodies, such as charcoal, hold much condensed air in their pores, and that probably they retain part of this even when reduced to powder, Professor Leslie obviates the chances of error arising from this source by comparing the dilatation which takes place under different degrees of pressure, under 10 inches and 20 for instance, or 71/2 and 15.

Charcoal, from its porosity, is so light that its specific gravity, as assigned in books, is generally under 0.5 less than half the weight of water, or one seventh the weight of diamond; taken in powder by the above instrument it exceeds that of diamond, is one half greater than that of whinstone, and is, of course, more than seven times heavier than has usually been supposed. Mahogany is generally estimated at 1.36; but mahogany sawdust proves by the instrument to be 1.68; wheat flour is 1.16"; pounded sugar 1.83; and common salt 2.15; the last agrees very accurately with the common estimate. Writing paper rolled hard by the hand had a specific gravity of 1,78, the solid matter present being less than one third of the space it apparently filled. One of the most remarkable results was with an apparently very light specimen of volcanic ashes, which was found to have a specific gravity of 4.4. These results are, however, given as approximations merely by the first instrument constructed.