GRAVITY

From Agepedia

GRAVITY (gravitas, Latin), in physics; the natural tendency or inclination of bodies towards a centre. Terrestrial gravity is that force by which all bodies are continually urged towards the centre of the earth. It is in consequence of this force, that bodies are accelerated in their fall, and, when at rest, that they press the body, or that part of the body, by which they are supported. As to the cause of gravity, or its nature, nothing is known; and it would be useless to detail the hypotheses advanced to account for this most important law of nature. All that can be said is, that it appears to be an essential property of matter, or, at least, of ai{ matter that has hitherto become the object of human investigation, though it is by no means certain that matter may not exist, which is not subject to its influence. This part of the subject appears to be beyond human comprehension. Instead, therefore, of wasting our time in useless speculation as to the cause, let us only attend to its effects, and content ourselves with examining more particularly the manner in which this principle operates on material bodies, and the laws by which it appears to be regulated ; the principal of which, as deduced from experiment, or from the most unequivocal inferences, are as follows: 1. that gravitation takes place between the most minute particles of bodies ; 2. that it is proportional to the masses of those bodies; 3. that it varies inversely as the square of the distance, in proceeding from the surface of the body outwards, or from its centre; 4. that it varies directly as the distance, in descending from the surface to the centre in uniform spherical bodies; 5. that it acts equally on bodies in a state of rest, as on those in motion, and that its action in the latter case is always the same, whether that motion be to or from the centre of attraction, or in any otlrer direction; 6. that it is transmitted instantaneously from one body to another. Gravity, as relating to the science of mechanics, is divided into absolute and relative. Absolute gravity is that by which a body descends freely and perpendicularly in a vacuum or nonresisting medium. Relative gravity is that by which a body descends, when the absolute gravity is constantly counteracted by a uniform, but inferior force, such as in the descent of bodies down inclined planes, or in resisting mediums. (See Inclined Plane.) Specific gravity 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, on account of its being less subject to variation in different circumstances of time, place, &c, than any other body, whether solid or fluid ; and, by a very fortunate coincidence, at least to English philosophers, it happens, that a cubic foot of rain water weighs 1000 ounces avoirdupois. 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 each in avoirdupois ounces, which is a great convenience in numerica1 computations. From the preceding defi nition, we readily draw the following laws of the specific gravity of bodies; viz. 1. in bodies of equal magnitude, the specific gravities are directly as the weights, or as their densities; 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 the compound ratio of their magnitudes and specific gravities. Hence it is obvious, that, of the magnitude, weight and specific gravity of a body, any two being given, the third may be found; and we may thus find the magnitude of bodies, which are too irregular to admit of the application of the common rules of mensuration ; or we may, by knowing the specific gravity and magnitude, find the weight of bodies which are too ponderous to be submitted to the action of the balance or steelyard; or, lastly, the magnitude and weight being given, we may ascertain their specific gravities. Other properties relating to the specific gravity of bodies are as follows ; viz. 1. A body immersed in a fluid will sink, if its specific gravity be greater than that of the fluid; if it be less, the body will rise to the top, and be only partly immerged; and if the specific gravity of the solid and fluid be equal, it will remain at rest in any part of the fluid in which it may be placed. 2. When a body is heavier than a fluid, it loses as much of its weight, when immersed, as is equal to a quantity of the fluid of the same bulk or magnitude. 3. If the specific gravity of the fluid be greater than that of the body, then the quantity of the fluid displaced by the part immerged, is equal to the weight of the whole body; and hence, as the specific gravity of the fluid is to that of the body, so is the whole magnitude of the body to the part immerged. 4. The specific gravities of equal solids, are as their parts immerged in the same fluid. 5. The specific gravities of fluids are as the weights lost by the same immerged solid. A solid substance, rarer than the fluid medium, must evidently sink, till it displace an equal weight of the fluid. The submerged 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 in stance, ana divided into portions wmcn or corfc mignt rest on tiie top or tlie correspond to the 1000th parts of the bulk alcohol.of the ball. Such is the general construe TMe , &, dJk Gmv{ties of Madg turn of the hydrometer, a very convenient ¦> JLL, £flrffe ^ instrument for examining readily the den PT ,_ o V . . , or" j'xx> .. v *J mi ... n lit may be convenient here to state merely in round sities of different liquids. The stem will numbers, the specific gravities of the more remarkscarcely bear more than 100 distinct sub able substances.] divisions; but the range can be easily Metals. enlarged, by attaching, as circumstances Platinum, purified, ..... 19.50 may require, loads answering to 100, 200, " hammered,..... 20.34 300, &c. One of the easiest and simplest " laminated,..... 20.34 methods of determining the densities of " drawn into wire, . . . 22.07 different liquids, is by a set of small glass Gold, pure and cast,..... 19.26 beads, previously adjusted, and numerical " hammered,...... 19.36ly marked. Thrown into any liquor, the Mercury,......... 13.57 heavier balls sink, till they approach the Lead, cast,........ 11.35 required density, and become gradually Silver, pure and cast,..... 10.47 buoyant, and the one which first rises to " hammered,...... 10.51 the surface indicates, in 1000th parts, the Bismuth, cast,....... 9.82 specific gravity of the fluid. These balls Copper, cast,....... 8.79 are adapted for examining liquids, wheth " wire,....... 8.89er lighter or heavier than water. But the Brass, cast,........ 8.40 most accurate and concise mode of ascer " wire,........ 8.54 taining the density of liquids, is to employ Cobalt and nickel, cast, .... 7.81a small glass measure with a very short, Iron, cast,......... 7.21 narrow neck, and adjusted to hold exactly Iron, malleable,....... 7.79 1000 grains of distilled water. The ves Steel, soft,........ 7.83 sel being filled with any other liquid, the " hammered, ...... 7.84 weight of it is observed, and thence its Tin, cast,......... 7.30 relative density to water may be found by Zinc, cast,........ 7.20 merely striking off three decimal places. Antimony, cast,....... 4.95At each operation, the glass must be care Molybdenum,....... 4.74 fully rinsed with pure water, and again Sulphate of baiytes,..... 4.43 dried, by heating it, and then sucking out Zircon of Ceylon,...... 4.41 the humified air, for a few minutes, by Stones. the help of a slender inserted tube. If Oriental ruby,....... 4.28 fluids of various densities, and not dispos Brazilian ruby,....... 3.53ed to unite in any chemical affinity, be Bohemian garnet,...... 4.19 poured into a vessel, they will arrange Oriental topaz,....... 4.01 themselves in horizontal strata, according Diamond,......... 3.50to their respective densities, the heavier Crude manganese,...... 3.53 always occupying a lower place. This Flint glass,........ 2.89 stratified arrangement of the several fluids Glass of St. Gobin, . . . . . 2.49 will succeed, even though a mutual at Fluor spar,........ 3.18 traction should subsist, provided its oper Parian marble, . . 2.34 ation be feeble and slow. Thus a body Peruvian emerald,...... 2.78of quicksilver may occupy the bottom of Jasper, . ........ 2.70a glass vessel, above it a layer of concen Eartlis, fyc. trated sulphuric acid, next this a layer of Carbonate of lime,...... 2.71 pure water, and then another layer of al Rock crystal,....... 2.65 cohol. The sulphuric acid would scarce Flint,.......... 2.59ly act at all upon the mercury, and a con Sulphate"of lime,...... 2.32 siderable time would elapse before the Sulphate of soda, ... . 2.20 water sensibly penetrated the acid, or the Common salt,...... 2.13 alcohol the water. Bodies of different Native sulphur,....... 2.03 densities might remain suspended in Nitre,.......... 2.00 those strata. Thus, while a ball of plati Alabaster,......... 1.87 num would lie at the bottom of the quick Phosphorus,........ 1 77 silver, an iron ball would float on its Plumbago,........ 1.86 surface; but a ball of brick would be Alum,.......... 172 lifted up to the acid, and a ball of beech Asphaltum,........ 1.40 would swim hi the water, and another Jet,........... 1.24 Coai, from.....1.24 to 1.30 geneous bodies, which may be divided Sulpnuric acid, ... ... 1.84 lengthwise into similar and equal parts, Nitric acid,........1.22 the centre of gravity will be the same as Muriatic acid, .......1.19 the centre of magnitude. The centre of Liquids, Oils, fyc. gravity of a parallelogram or cylinder, or Equal parts by weight of water and any prism whatever, is in the middle point alcohol,..........93 of the axis, and the centre of gravity of a Ice,............92 circle, or any regular figure, is the same Strong alcohol,........82 as the centre of magnitude. The cornSulphuric ether,........74 mon centre of gravity of two bodies, is a Naphtha,..........71 point so situated in a right line joining the Sea water,......... 1.03 centres of the two bodies, that, if the point Oil of sassafras,.......1.09 be suspended, the two bodies will equi Linseed oil,.........94 ponderate and rest. Thus the point of Olive oil,..........91 suspension in a balance or steelyard, where White sugar, ........1.61 the two weights equiponderate, is the com Resins, Gums, frc. mon centre of gravity of the two weights. Gum arabic and honey, .... 1.45 Gravity, in music, is the modification pitch,..........1.15 °f any sound, by which it becomes deep Isinglass,.........1.11 or low in respect of some other sound. Yellow amber........1.08 GRAY, Thomas, a distinguished English Hen's egg, fresh laid,' ' .' .' .' .* 1.09 Poet> was the s°n of a money scrivener in Human blood .......1.05 the city of London, where he was born in Camphor, .'.!!!!!!! .99 1716. He was sent to Eton, and there laid White wax .........97 the foundation of his future intimacy with Tallow........1 . .94 Horace Walpole and Richard West. In pear] ..........275 1 734, he removed to Cambridge as a stu Sheep's bone..... ! 2.22 dent of St. Peterhouse, where he early ob Ivory . ..!..!!. 1.92 tained some reputation for literature and Ox's horn, ...!!!!!. 1.84 poetry. He quitted college in 1738, andIV d...... entered himself at the Inner Temple, with Liffnum vita* °° ' 1 T2 a v*ew of studying law, but was easily in Fbonv '........118 duced to accept the invitation of Mr. Wal Mahoe'anv "***""*"" l'ofi P°le to accompany him in his tour of Eu Drv oak .........°/^ roPe> towards the close of which theyr" J i '..........ne separated, in consequence of some dis* i '...........QA agreement. Gray finished the expedi Eim, from: : : ::: : .Wto^o ^1*^™^,^ Pit. iw,™ s7 tr, *n m 1^41. His father soon after died, and Poplar ......' 38 leavinS but a sma11 property, Mr. GrayQ v '...........04 returned to academic retirement at Cam'............ bridge. Here he occupied himself several Gases. years in laying literary schemes and plans Chlorine, .........00302 of magnitude, which he admirably com Carbonic acid gas,......00164 menced, but wanted energy to mature. Oxygen gas, ........09134 So slow was he to publish, that it was not Atmospheric air, ...... .00121 until 1747 that his Ode on a distant ProsAzote, ..........00098 pect of Eton College made its appearance; Hydrogen gas, .......00008 and it was only in consequence of the Gravity, Centre of, in mechanics, is a printing of a surreptitious copy, that, in point within a body, through which, if 1751, he published his Elegy written in aa plane pass, the segments on each side Country Churchyard. In 1757, on the will equiponderate; that is, neither of them death of Cibber, the office of laureate was can move the other. Hence, if the de offered to Mr. Gray, who declined it, and scent of the centre of gravity be prevented, the same \ear published his two principalor if the body be suspended by its centre odes, On the Progress of Poesy, and Theof gravity, it will continue at rest in equi Bard. In 1759, he removed to London, librium in any position. The whole grav where he resided for three years. In 1768, ity, or matter, of a body may be conceived the duke of Grafton presented him with united in its centre of gravity ; and, there the professorship of modern history at fore, it is usual, in demonstration, to sub Cambridge ; in consequence of which he etitute the centre for the body. In homo wrote the Ode for Music, for the installa tion of that nobleman as chancellor of the university the following year. It was the intention of Gray to do something more than his predecessors, who had made the office a sinecure, although affording a salary of 300Z. per annum ; but, his health soon after declining, he proceeded no farther than to sketch a plan for his inaugu^ ration speech. He died of the gout in his stomach, on the 30th July, 1771, in his fiftyfifth year, and was buried with his mother in the churchyard of Stoke Pogeis in Buckinghamshire. As a poet, Gray is splendid, lofty, energetic and harmonious. Although lyric poetry was what he chiefly cultivated, he would have excelled in the didactic, if a judgment may be formed from his noble fragment of An Essay on the Alliance of Education and Government. As a writer of Latin verse, he is surpassed by few, and his letters are admirable specimens of the epistolary style. In his disposition he was peculiarly fastidious, which gave an air of effeminacy and timidity to his manners, subjecting him to much ridicule, at the same time singularly contrasting with the manly strains of his poetry. His general acquirements were uncommon, but his want of energy and perseverance rendered his extensive research little effective. (See Memoirs of his life, &c. by Mason.)