GRAVITY, in Physics, the natural tendency of bodies towards • 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 eircumterrestrial bodies tend towards a point which is either accurately or nearly die centre of magnitude of the terraqueous globe. 2. In all places equidistant from the centre of the earth the force of gravity, cetera paribas, 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 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.
Geeeral, 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, apeafic, 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 fbot of such bodies in avoir dupoise ounces. From the preceding definition may be drawn the following
laws of the specific gravity of bodies. I. 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. 6. 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 fame fluid of equal bulk. 8. 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 • " 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 sub stance is immersed in water causes their specific gravity when ascer tained by the hydrostratic balance to appear lees than it really is.