524 



GRAVITY GRAY. 



lost by the same unmerged solid. A solid substance, 

 rarer than the fluid medium, must evidently sink, till 

 it displace an equal weight of the fluid. The sub- 

 merged part of the solid hence always marks the 

 volume of this equiponderant mass. If the floating 

 body have a globular shape, terminated by a long 

 slender stem, its depression in any liquid will measure 

 the smallest differences of specific gravity. The stem 

 may be made exactly cylindrical, for instance, and 

 divided into portions which correspond to the 1000th 

 parts of the bulk of the ball. Such is the general 

 construction of the hydrometer, a very convenient 

 instrument for examining readily the densities of 

 different liquids. The stem will scarcely bear more 

 than 100 distinct subdivisions ; but the range can be 

 easily enlarged, by attaching, as circumstances may 

 require, loads answering to 100, 200, 300, &c. 

 One of the easiest and simplest methods of determin- 

 ing the densities of different liquids, is by a set of 

 small glass beads, previously adjusted, and numeri- 

 cally marked. Thrown into any liquor, the heavier 

 balls sink, till they approach the required density, 

 and become gradually buoyant, and the one which 

 first rises to the surface indicates, in 1000th parts, the 

 specific gravity of the fluid. These balls are adapted 

 for examining liquids, whether lighter or heavier than 

 water. But the most accurate and concise mode of 

 ascertaining the density of liquids, is to employ a 

 small glass measure with a very short, narrow neck, 

 and adjusted to hold exactly 1000 grains of distilled 

 water. The vessel being filled with any other liquid, 

 the weight of it is observed, and thence its relative 

 density to water may be found by merely striking 

 off three decimal places. At each operation, the 

 glass must be carefully rinsed with pure water, and 

 again dried, by heating it, and then sucking out the 

 humified air, for a few minutes, by the help ofa slender 

 inserted tube. If fluids of various densities, and not 

 disposed to unite in any chemical affinity, be poured 

 into a vessel, they will arrange themselves in hori- 

 zontal strata, according to their respective densities, 

 the heavier always occupying a lower place. This 

 stratified arrangement of the several fluids will suc- 

 ceed, even though a mutual attraction should subsist, 

 provided its operation be feeble and slow. Thus a 

 body of quicksilver may occupy the bottom of a 

 glass vessel, above it a layer of concentrated sul- 

 phuric acid, next this a layer of pure water, and then 

 another layer of alcohol. The sulphuric acid would 

 scarcely act at all upon the mercury, and a consider- 

 able time would elapse before the water sensibly 

 penetrated the acid, or the alcohol the water. Bodies 

 of different densities might remain suspended in those 

 strata. Thus, while a ball of platinum would lie at 

 the bottom of the quicksilver, an iron ball would float 

 on its surface ; but a ball of brick would be lifted 

 up to the acid, and a ball of beech would swim in the 

 water, and another of cork might rest on the top of 

 the alcohol. 



Table of Specific Gravities of Metals, Stones, 

 Earths, fyc. 



[Itiray be 



.venieitt here to state merely the specific gravities of the more 

 remarkable substances.] 



Gravity, Centre of, in mechanics, is a point within 

 a body, through which, if a plane pass, the segments 

 on each side will equiponderate ; that is, neither of 

 them can move the other. Hence, if the descent of 

 the centre of gravity be prevented, or if the body be 

 suspended by its centre of gravity, it will continue at 

 rest in equilibrium in any position. The whole 

 gravity, or matter, of a body may be conceived 

 united in its centre of gravity ; and, therefore, it is 

 usual, in demonstration, to substitute the centre for 

 the body. In homogeneous bodies, which may be 

 divided lengthwise into similar and equal parts, the 

 centre of gravity will be the same as the centre of 

 magnitude. The centre of gravity of a parallelogram 

 or cylinder, or any prism whatever, is in the middle 

 point of the axis, and the centre of gravity of a 

 circle, or any regular figure, is the same as the centre 

 of magnitude. The common centre of gravity of two 

 bodies, is a point so situated in a right line joining the 

 centres of the two bodies, that if the point be sus- 

 pended, the two bodies will equiponderate and rest. 

 Thus the point of suspension in a balance or steel- 

 yard, where the two weights equiponderate, is the 

 common centre of gravity of the two weights. 



Gravity, in music, is the modification of any sound, 

 by which it becomes deep or low in respect of some 

 other sound. 



GRAY, THOMAS, a distinguished English poet, 

 was the son of a money scrivener in the city of Lon- 

 don, where he was born in 1716. He was sent to 

 Eton, and there laid the foundation of his future in- 

 timacy with Horace Walpole and Richard West. In 

 1734, he removed to Cambridge as a student of St 

 Peterhouse, where he early obtained some reputation 

 for literature and poetry. He quitted college in 1738, 

 and entered himself at the Inner Temple, with a view 

 of studying law, but was easily induced to accept the 

 invitation of Mr Walpole to accompany him in his 

 tour of Europe, towards the close of which they sepa- 

 rated, in consequence of some disagreement. Gray 

 finished the expedition by himself, and returned to 

 England in 1741. His father soon after died, and 

 leaving but a small property, Mr Gray returned to 

 academic retirement at Cambridge. Here he occu- 

 pied himself several years in laying literary schemes 

 and plans of magnitude, which he admirably com- 



