weight. By dropping the materials between the planks from some height they pack more closely than if gently laid in. As the wall reaches the upper edge of the board or plank casing, another tier of the same is added, and when this space is filled up, the lower tier is taken off and added to the upper; and so they are alternately raised one over the other, till the wall has reached the height required. The casing is prevented from spreading by being fastened to sticks laid across the wall, which may be covered over and left in. Frames for doors and windows are set in their places, whenever the wall reaches the level on which they are to stand. The work should not be interrupted at any time so long as to permit the upper layers to set and become dry, and care should be taken not to remove the casing at the bottom before the wall has hardened sufficiently to support itself. During the progress of the work it is essential to protect it from the rain; and it is well to continue this care for some days after the materials appear to have assumed a solid state. Many cases have occurred of the walls of a house being completely washed down in consequence of neglect of this precaution. When finished they should be sheltered by projecting eaves. If to be stuccoed, more care should be taken to keep the outside smooth and even than if it is to be left uncovered. A sufficient thickness for walls 25 feet high is 16 inches for the lower half and 10 or 12 for the upper. A house of this description, the construction of which was observed by the writer, of octagonal form, measuring 16 feet on a side, or 128 feet in circumference, was raised by the labor of 3 men at the rate of one foot a day. The wall was 14 inches thick; the proportion of lime, unusually large, was of the whole. The total cost, including the materials, was estimated at $12 per foot in height; or, making no deduction for spaces for doors and windows, 8 cents per cubic foot.

GRAVES, a S. W. county of Ky., bordering on Tennessee, and drained by Mayfield creek and Obion river; area, 515 sq. m.; pop. in 1850, 11, 397, of whom 1,439 were slaves. The surface is level, and the soil generally productive. In 1850 it yielded 653,838 bushels of Indian corn, 15,036 of wheat, 115,979 of oats, 1,090,545 lbs. of tobacco, 17,657 of wool, and 10,982 of flax. There were 17 corn and flour mills, 6 saw mills, 1 woollen and 4 cotton factories, 33 churches, and 1,150 pupils attending public schools. Value of real estate in 1855, $1,512,344. Formed in 1823, and named in honor of Major Benjamin Graves, who was slain at the battle of the river Raisin. Capital, Mayfield. The New Orleans and Ohio railroad, when completed, will pass through this county.

GRAVESANDE, WILLEM JACOB VAN 's, a Dutch philosopher and mathematician, born in Bois-le-Duc, Sept. 27, 1688, died in Leyden, Feb. 28, 1742. He published at the age of 18 an essay on perspective, which was applauded by Bernouilli. At the same time he published

a philosophical thesis on suicide. After completing his studies in the university of Leipsic in 1707, he was admitted to the bar at the Hague, where he wrote for the "Literary Journal" an examination of Fontenelle's "Geometry of the Infinite," a dissertation on the construction of the air pump, one concerning the force of bodies, in which he embraced the opinion of Leibnitz against that of Newton, which he had formerly defended, and dissertations upon the motion of the earth, &c. In 1715, being sent as secretary of legation to London to congratulate George I. on his accession to the throne, he was there admitted a member of the royal society. In 1717 he was appointed professor of mathematics and astronomy in the university of Leyden, and exchanged his chair in 1734 for that of philosophy, which he held till his death. His philosophical writings are marked by the precision to which mathematical studies had habituated him; but being unable to decide between the doctrines of Locke, Descartes, and Leibnitz, he borrowed ideas from each. He was the first to introduce the theories of Newton upon the continent. His principal works are: Physices Elementa Mathematica (2 vols. 4to., the Hague, 1720-'42); Matheseos Universalis Elementa (8vo., Leyden, 1727); Introductio ad Philosophiam, Metaphysicam, et Logicam (Leyden, 1736-7).

GRAVESEND, a municipal borough, town, and river port of England, in the co. of Kent, on the right bank of the Thames, 21 m. S. E. of London, with which it is connected by railway; pop. in 1851, 16,633. The principal public edifices are the town hall, parochial church, where Pocahontas is buried, literary institution, theatre, &c. Ship building is carried on to a considerable extent, but the chief trade arises from supplying outward bound ships with stores and clothing.

GRAVIŇA, GIOVANNI VINCENZO, an Italian jurist and man of letters, born in Roggiano, Jan. 20, 1664, died in Rome, Jan. 6, 1718. While studying law at Naples he perfected himself in the Greek language, commenced essays on poetry, and composed two dramas. Subsequently he devoted himself to the civil and the canon law, went to Rome in 1689, published several brief works on morals and literature, and in 1695, having collected 15 of his friends in his garden, organized them into the celebrated academy of the Arcadians. Pope Innocent XII. offered him the highest ecclesiastical honors, but he refused to enter the priesthood. In 1699 he was appointed to the chair of civil law in the college of La Sapienza, which he exchanged in 1703 for that of the canon law. He soon after published his works on the "Origin of the Civil Law" and on the "Roman Empire." On account of the literary jealousies of the time a schism took place in 1711 in the academy of the Arcadians, and Gravina and his friends withdrew and founded the academy of the Quirina. Gravina was the adoptive father of Metastasio, whom he made his principal heir.

GRAVITY, or GRAVITATION, the mutual attraction of bodies of matter. From the earliest times men were familiar with two sets of simple and unvarying phenomena-the fall of heavy bodies set free above the earth's surface, and the pressure of such bodies on the surface or on any support. These facts, equally applicable to their own physical organization, they must early have roughly generalized into laws. Another set of facts, apparently very different, speedily claimed and received attention. Thousands of years before the principles of the inductive philosophy were formally stated, or its fruits began in any large sense to be attained, curiosity led patient observers to chronicle the phenomena that could alone pave the way for the distant yet possible discoveries of natural law. The places, paths, and times of the heavenly bodies were so correctly made out and recorded, that, even before the true cause of their movements had been conjectured, successive erroneous views of those movements had given way before the true, heliocentric theory. It was not until about the close of the 16th century of our era that the work of properly analyzing and explaining these stores of fact was commenced. Kepler began, in one direction, to prepare the way by includ' ing a vast amount of the results of previous astronomical observation in the three laws which bear his name. In the other direction, every language had, of course, its name for that manifestation of downward pressure of bodies near the earth which we call weight. With the Latins this tendency was named gravitas, whence our term gravity. Archimedes studied specific gravity, and determined the centre of gravity, empirically, and without arriving at their cause. Galileo first successfully investigated the cause or force producing gravity of bodies near the earth's surface, showing that this force is a continual or constant one, and deducing the rate of acceleration of the motion caused by it. The idea of a centripetal force, by which the planets would tend to the sun, seems at the same time to have been slowly forming itself in the minds of philosophers. Kepler spoke of an immaterial virtue or magnetic nature of the sun; but he explained the planetary motions rather by an existing vortex of fluid in which they were carried along, than by a present, acting energy. Descartes supposed a number of these vortices corresponding to the number of the planets. Gassendi seems to have ascribed the motion to traction by fibres, like those of muscles; Leibnitz, to agitations of an ether. Borelli, in 1666, seems to have anticipated Newton, though without demonstrating the truth that the central orb "draws and holds" those revolving around it; while between this drawing and a tendency to recede from their centre of revolution, the latter would be kept in their orbits. In England, Gilbert, Boyle, Horrocks, Wren, Halley, and Hooke, and in Holland, Huyghens, approached more or less clearly the idea of a central force acting toward the

sun; and by some of these observers the true rate of decrease of the attractive force with distance was understood. Wren, Hooke, and others, contested Newton's priority in the discovery, of the truth of which, however, none of them had furnished the required mathematical and deductive proof. The credit due to Sir Isaac Newton, then, is not that of having discovered the principle of gravity, which in a degree all men understood; nor of having first imagined a central force acting toward the sun, nor the ratio of its diminution with increase of distance; and perhaps not even of having been the first to conjecture the identity of this central force with terrestrial gravity. But the steps by which he set out to prove or disprove the truth of these conceptions, and the results which he deduced from the truth when determined, sufficiently attest his claim as the substantiator, and therefore, in the ethics of science, the discoverer (for the field of the solar system), of the law of gravitation—“ the greatest scientific discovery ever made." In Kepler's second law, that the radius vector of the orbit of a planet sweeps over equal areas in equal times, Newton found proof of the necessity of a central force acting toward the sun; in the first law, that the orbits are ellipses, he found the confirmation of the principle of diminution of action of the force in the ratio of the inverse square of the distance; and from the third law, that the squares of the periodic times of the planets are as the cubes of their mean distances from the sun, he inferred that the same force, diminishing in such ratio, must pervade the entire system. Here, then, was a complete law of planetary gravitation; though the cases of perturbations, of the satellites, and of falling bodies at the earth, were not yet included. To link these with the former, he commenced with the case of the earth and her moon. The story that Newton was first led to this essential step, during his retreat from Cambridge in 1666, the summer of the plague, by reflections excited upon witnessing the fall of an apple from a tree under which he was sitting, although discredited by Brewster, is reaffirmed by a more discriminating writer, Biot, who cites the confirmation of the story by Mr. and Mrs. Conduit, the latter the niece of the philosopher, and the fact that Pemberton used to point out the very garden in which the tree stood. We may safely leave to the orators their favorite illustration, when we reflect that no mind, not already busied with the problem, and far advanced in the analysis of its conditions, could have perceived the significance of so humble and common a phenomenon. By some train of thought, it is certain, the philosopher was led to inquire why the force acting from the earth should cease at the height from which bodies are known to fall; why at 10, 100, or 1,000 miles; and why it should not act even at the distance of the moon. If it did, the moon should be incessantly falling toward the earth, and by an amount as much less than the fall of a body near the earth

as the square of the distance of such body from the earth's centre is less than the square of the distance between the centres of the moon and earth. Now, the fall or deflection of the moon from a line tangent to any part of her path, as already deduced from observation, was known to be about 15 feet per minute; while near the earth a body falls 16X602-57,960 feet, in the same time. The moon's distance, as determined by her parallax, varies in different parts of her orbit, and at different parts of the earth's surface, from about 56 to about 64 times the length of the earth's radius, or semi-diameter. Newton assumed the distance as at that time deduced from the supposed radius of the earth; but the result of his calculation gave a fall of 13 feet only; and unsatisfied with such an approximation, he dropped the subject for many years. In the mean time, Picard having executed a more accurate measurement of a portion of a meridian, the length of the earth's radius, and as a consequence the distance of the moon, became more correctly known; and learning these facts, Newton in 1682 resumed his calculations. Taking the moon's distance at 61 times the earth's radius, he found that 57,960 feet+612 gave the gratifying result of a fall of a little more than 15 feet per minute at the moon, an amount almost exactly coincident with that known through actual observation to occur, and which should have been found if the estimates were correct, and the force acting upon the moon were the same as that operating on terrestrial bodies. Thus was established the cosmical character of the force of gravitation. Verifications for the satellites of the other planets, and the perturbations of the bodies of our system, rapidly followed. Subsequently, the tides, the figure of the planets, the precession of the equinoxes, and the revolutions of comets, were brought under the same law. The complex movements of the planetary bodies, which for ages had perplexed the profoundest minds, now at once became, through an understanding of the simple laws of motion and of that of the gravitation of all bodies within the system toward each other, comprehensible by every pupil of ordinary ability. But yet, again, Newton had not discovered universal gravitation; and only by conjecture could his law be applied to bodies beyond our solar system. Newton had correctly inferred, however, that gravitation is a force acting between all bodies, and even all the molecules of bodies, so far as it can be known to extend; and that it is proportional in every case to the mass of matter acted upon. A few important experiments may be named which have since served to corroborate the law in these particulars. In 1774 Maskelyne suggested an observation of the effect of the mass of Schehallion, a mountain in Scotland, upon the plumb line; the result was the discovery of the deviation of the plummet toward the mountain to the extent of nearly 6" of a degree. Observations made by Bouguer and others upon the influence of other mountain masses have since confirmed this re

sult. About the same time, by the use of a delicate torsion balance, in which the force of attraction was determined by the amount of twist produced in a slender suspending wire, Cavendish showed the ratio of the attraction of known masses to that of the earth as a whole, and also the equal attraction of equal masses of whatever kind. But doubts having still been suggested in regard to this part of the law, Bessel more recently took up the subject and in 1832 published results showing that for every substance examined, including not only those ordinarily known, but also meteoric stones and metals, the gravitating force was exactly proportional to the inertia, and hence to the quantity of matter. It was reserved for the two Herschels to carry the law beyond the limits of our system, by showing its operation in the instances of double and multiple stars, the components of several of which have been found to have a mutual revolution in ellipses about each other, and some of theso in known times varying between 30 and 608 years; and thus to render the law a universal one. The observations of Madler and others in reference to a supposed rotation of our distinct stellar system about some centre, probably at or near the star Alcyone of the Pleiades, promise further to confirm this view. In its present form, then, the law of gravitation, probably the broadest inductive generalization yet reached in physical science, may be thus stated: All bodies, of whatever kind, at finite distances, are incessantly attracted toward each other with a force that, between any two of them, is mutual and equal, and acting in the direction of a line joining the centres of their masses, and that, in magnitude, is directly as the masses, and inversely as the squares of the distances between their centres.-A few points remain to be named. Gravitation, the most feeble of physical actions, is between small masses almost imperceptible; yet it is an energy abundant in proportion to the quantity of matter in the universe, and fully competent, by its gradual condensing agency, to account for the origination of planetary systems and their movements. It is not strange, therefore, that by some physicists this energy is beginning to be supposed to be that of which all other forms of force are residues or metamorphoses. Gravity is the name especially given to its terrestrial manifestation. A particle or body without a sphere or spheroid, solid or hollow, is attracted to the centre of the mass of such body; within a hollow sphere, it will remain at rest at any point. At different depths below the earth's surface, a body will be attracted with a force diminishing as the distance from the centre decreases. The slight variation in the gravitating force of the same falling body at different heights is in practice usually disregarded. The weight of a body, as the measure of its gravitating tendency, must vary both with mass and with the force acting on it; hence, from the form of the earth, the same body at the sea level will weigh less and

less as it is removed from either pole toward the equator. An elevation above the sea level gives a like result. A stone falls through a less distance in a given time on a mountain than in the valley below, less at the equator than at either pole; and the oscillations of a given pendulum, under the same circumstances, are less rapid in a similar degree. The loss of weight in these cases cannot be tested by ordinary scales or steelyards, in which this loss is equal on both sides; but it may be by the spring balance, in which bodies are weighed by the pull they exert against the elasticity of a coiled wire. The effect of centrifugal force, increasing from the pole to the equator, cooperates with increasing removal from the earth's centre to lessen weight; the result of the combined action of these two causes is, that a body weighing 195 lbs. at either pole will weigh but 194 over the equator. The line of a falling body, called also the line of direction, is interesting as being that direction in space at any point of the earth's surface with reference to which all other directions are named, and by which they are to be determined.

GRAVITY, SPECIFIC, the proportion of the weight of a body to that of an equal volume of some other substance adopted as a standard of reference. For solids and liquids the standard is pure water, at a temperature of 60° F., the barometer being at 30 inches. Aeriform bodies are referred to the air as their standard. A cubic foot of water weighing 1,000 ounces, if the same bulk of another substance, as for instance cast iron, is found to weigh 7,200 ounces, its proportional weight or specific gravity is 7.2. It is convenient to know the figures representing this proportion for every substance in common use, that the weight of any given bulk may be readily determined; and for all substances the specific gravity is used among other tests for the purpose of distinguishing bodies from each other, the same substance being found, under the same circumstances of temperature, &c., to retain its peculiar proportional weight or density. Hence tables of specific gravities of bodies are prepared for reference, and in every scientific description of substances the specific gravity is mentioned. In practical use, the weight of a cubic foot is obtained from the figures representing the density by moving the decimal point 3 figures to the right, which obviously from the example above gives the ounces, and these divided by 16 the pounds avoirdupois, in the cubic foot. Different methods may be employed to ascertain the specific gravity of solids. That by measuring the bulk and weighing is rarely practicable, nor is it desirable. As a body immersed in water must displace its own bulk of the fluid, the specific gravity may be ascertained by introducing a body, after weighing it, into a suitable vessel exactly filled with water, and then weighing the fluid which is expelled. The proportional weight is then at once obtained. Wax will cause its own weight of water to overflow; its specific gravity is then 1. Platinum, according to the condition it is in, will

cause only from to of its weight of water to escape, showing its specific gravity to be from 21 to 21.5. But a more exact method than this is commonly employed. The difference of weight of the same substance, weighed in air and when immersed in water, is exactly that of the water it displaces, and may consequently be taken as the weight of its own bulk of water. The specific gravity then is obtained by weighing the body first in air, and then, suspended by a fibre of silk or a hair, in water, and dividing the weight in air by the difference. It is hardly necessary to say that the substance examined must be free from mixture of foreign matters, and especially from cavities that may contain air. Minerals, if suspected to contain such, should be coarsely pulverized, and then the second method above may be conveniently applied to determine their density. Thus prepared, a higher result will be obtained, and even metals when pulverized were found by Rose to give a greater specific gravity than when this is determined from samples in their ordinary state. Very fine powders may also be determined by the method in use for ascertaining the specific gravity of fluids, viz.: by comparing the weight of a measured quantity with that of the same quantity of water. A glass vessel called a specific gravity bottle is commonly employed, which is furnished with a slender neck, upon which is a mark indicating the height reached by 1,000 grains of water. The substance to be examined is introduced till it reaches the same mark, and, the weight of the empty bottle being known, only one weighing is required to obtain the result. The simple method sometimes adopted for testing the purity of gold coin is similar to this. If the coin is adulterated, it is with a lighter metal. It is first weighed against a genuine coin, and if right in this respect, it is next applied to a slit in which the genuine coin just fits; any difference of bulk, either in thickness or diameter, is readily detected, and shows the piece to be false. The specific gravity of fluids is also taken by the instrument called a hydrometer or areometer, of which several are in use, slightly differing in construction, but all dependent on the principle that the weights required to immerse a light body, as a bulb of glass, in different fluids, are proportional to the densities of these fluids. Such instruments are much used for ascertaining the specific gravity of spirituous and other liquors, as an indication of their strength. (See AREOMETER, and HYDROMETER.) If the solid body to be tested is lighter than water, it must be attached to some heavy substance to cause it to sink. Its specific gravity is then calculated by dividing its weight in the air by the sum of the weights of the attached body both in air and in water, first subtracting from this sum the weight of the two bodies together in the water. Bodies soluble in water may be weighed in some other fluid, as alcohol, ether, olive oil, &c., and their proportional weight to that of this fluid being thus ascertained, their