GRA
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GRA
'Tis cured bx pulling off the Shoe, drawing the Place to the Quick, picking out all the Gravel, and flopping up the Foot with Horfe-greafe, and Turpentine pour'd in hot.
GRAVER, a Steel Inftrument, ferving to engrave on Metals. Sec Engraving.
The Graver confifts of four Sides, or'Faces ; and the Point ufually terminates in a Lozange. The other End is fitted into a Wooden Handle. See Steel, Temper, Sec.
Befide Engravers, the Seal-cutters, Lockfmiths, Gun- fmiths, Goldfmiths, Armourers, Spurmakers, &c. likewife make ufe of Gravers.—
GRAVING, the Aft, or Art of cutting Lines, Figures, and other Defigns on Metals ; more properly called En- graving. See Engraving.
GRAVITATION, the Exercife of Gravity, or the Pref- fure a Body exerts on another Body underneath it by the Power of Gravity. See Gravity.
'Tis one of the Laws of Nature, difcover'd by Sir Jfaac Newton, and now received by molt Philofophsrs, that every Particle of Matter in Nature gravitates towards every other Particle ; which Law is the Hinge whereon the whole New- tonian Philofophy turns. See Newtonian 'Philofophy.
What we call Gravitation, with refpect to thcGravitating Body, is call'd Attraction with refpect to the Body gravitated to. See Attraction.
The Planets, both primary and fecundary, as alfo the Co- mets, all gravitate toward the Sun, and toward each other; and the Sun toward them ; and that in proportion to the Quantity of Matter in each. See Planet, Comet, Sun, Earth, Moon, &c
The Peripateticks, &c. hold that Bodies only gravitate ■when out of their Natural Places ^ and that Gravitation ceafes when they are reftored to the fame ; the Intention of Nature being then fulfill'd. The final Caufe of this Faculty, they hold, is only to bring Elementary Bodies to their proper Places where they may reft. But the Moderns fhew that Bo- dies exercife Gravity even when ar reft, and in their proper Places.
This is particularly fhewn of Fluids $ and it is one of the Laws of Hydroftaticks, demonflrated by Mr. Soy le, and o- thers, that Fluids gravitate in propria toco, the upper Parts preffing on the lower, &c. See Fluid.
For the Laws of Gravitation of Bodies in Fluids, fpecifi- ealiy lighter or heavier, than themfelves ^ fee Specific Gravity.
GRAVITY, in Phyficks, the natural Tendency, or In- clination of Bodies towards a Centre. See Centre.
In this Senfe Gravity coincides with Centripetal Force. See Centripetal Force.
Gravity is defined by others, more generally, to be the Natural Tendency of one Body towards another j and by other more generally ftill, the mutual Tendency of each Body, and each Particle of a Body towards all others: In which Senfe the Word coincides with what we more ufually call Attra- Bion. See Attraction.
The Terms Gravity, Centripetal Force, Weight, and At- traction, do, in effedt, all denote the fame thing 5 only in different Views, and Relations : Which different Relations, however, Authors are apt to confound ; and accordingly, the four, are frequently ufed promifcuoufly.
In Propriety, when we confider a Body as tending towards the Earth, the Force wherewith it tends we call Gravity, Force of Gravity, or Gravitating Force: The fame Force, when wc confider it as immediately tending to the Centre of the Earth, we call Centripetal Force: The fame, when we confider the Earth, or Body toward which it tends, we call Attraction, or Attractive Force ; and when we confider it, in refpect. of an Obftacle, or a Body in the way of its Ten- dency, upon which it acts, we call rtWeight. See Weight, &c.
Philofophers conceive very differently of Gravity : Its Na- ture, Ufe, Phenomena, Caufe, Effects, and Extent have afforded various Speculations in all Ages.
Some confider it as an active Property, or innate Vis in Bodies, whereby they endeavour to join their Centre. O- thers hold Gravity in this Senfe, to be an occult Quality, and as fuch to be exploded out of all found Philofophy. See Occult Quality.
Sir If. Newton, tho' he frequently calls it a Vis, Power, or Property in Bodies -, yet explains himfelf, that all he in- tends by the Word, is the Effect, or the Phenomenon. He does not confider the Principle, the Caufe whereby Bodies tend downwards, but the Tendency itfelf, which is no occult Quality, but a fenfible Phenomenon, be its Caufe what it will, whether a Property effential to Body, as fome make it; or fuperadded to it, as others 5 or even an Impulfe of fome Body from without, as others. See Newtonian ^Philofophy.
'Tis a Law of Nature long obferv'd, that all Bodies near the Earth, have a Gravity, or tendency towards the Cen- tre of the Earth ; which Law, the Moderns, and particu- larly the immortal Sir Jf. Ne-wton, have found, from certain
Obfervation, to be much more Extcnfive ; and to hold u- niverfally with refpect to all the known Bodies in Nature- See Nature.
'Tis now, therefore acknowlcdg'd a Principle or Law of Nature, that all Bodies, and all the ^Particles of all Sodies gravitate towards each other, mutually : From which fingle Principle, Sir If. Newton has happily dedue'd all the great Phenomena of Nature. See Newtonian Philofophy.
Hence Gravity may be diftingui/h'd into, 'Particular, and General.
'Particular Gravity is that whereby heavy Bodies defcend towards the Earth. See Earth.
'Phenomena, or 'Properties of 'Particular Gravity.
i° All Circumterreftrial Bodies do hereby tend towards a Point which is either accurately, or very nearly the Centre of Magnitude of the terraqueous Globe. See Terraque- ous Globe.
z Q This Point, or Centre is fix'd within the Earth 5 or at leaft hath been fo ever fince we have had any authentick Hiffory:
For a Conference of its (hifring, tho 1 ever fo little, would be the overflowing of the low Lands, on that Side of the Globe towards which it approached. Dr. Halley fuggefis, it would well account for the univerfal Deluge, to have the Centre of Gravitation removed for a Time towards the Middle of the then inhabited World : For the Change of Place but the 2000 th Part of the Radius of our Earth would be fufficient to lay the Tops of the higheft Hills under Wa- ter. See Deluge.
3 In all Places equi-diflant from the Centre of the Earth, the Force of Gravity is nearly equal.
Indeed all Places of the Earth's Surface are not at equal Diftances from the Centre 5 becaufe the Equatorial Parts are fomething higher than the Polar Parts; The Difference be- tween the Earth's Diameter and Axis being about 34 Eng- lijb Miles, which hath been proved by the Necefiity of mak- ing the Pendulum fhortcr in thofe Places, before it will fwing Seconds. See Pendulum.
4 Gravity equally affects all Bodies, without regard ei- ther to their Bulk, Figure, or Matter: So that abstracting from the Refiftance of the Medium, the moft compact and loofe, the greateft and fmalleft Bodies would defcend equal Spaces in equal Times; as appears from the quick Defcent ot very light Bodies in the exbaufted Receiver. See Va- cuum.
Hence, a very great Difference may be obferved betwixt Gravity and Magnetifm 5 the latter affecting only Iron, and that towards its Poles 5 the former, all Bodies alike, in every Part. See Magnetism.
Hence, it follows that Gravity, in all Bodies, is propor- tionable to their Quantity of Matter • fo that all Bodies con- fift of Matters equally heavy. See Body, Matter, &e.
Hence alfo may be concluded that there is no fuch thing as pofitive Levity, thofe Things which appear light, being only comparatively fo. See Levity.
If feveral things afcend, and fvvim in Fluids, it is only be- caufe they are not Bulk for Bulk, fo heavy as thofe Fluids ; Nor is there any Reafon, why Cork, for inltance, iliould be faid to be light, becaufe it Iwims on Water, any more than Iron, becaufe it will fwim on Mercury. See Specif icGra- vity, &c.
5 This Power increafes as we defcend, and decreafes as wc alcend from the Centre of the Earth, and that in the pro- portion of the Squares of the Diffances therefrom recipro- cally ; fo as, for inltance, at a double Diitance, to have but a Quarter of the Force, ££c.
6° As all Bodies gravitate towards the Earth 5 fo, does the Earth equally gravitate towards all Bodies, i.e. The Action of Gravity is mutual on each Side, and equal. See Reaction.
Hence alfo the attractive Powers of Bodies, at equal Di- ftances from the Centre, are as the Quantities of Matter in the Bodies.
Hence alfo the attractive Force of the entire Bodies, con- fifts of the attractive Force of the Parts : For by the adding, or taking awayany Part of the Matter of a Body, its Gravity is increas'd, or diminiffi'd in the Proportion of the Quantity of fuch Particle to the entire Mafs. —
General, or Univerfal Gravity.
TheExiftence of the fame Principle of Gravity m the great Regions of the Heavens, aswell as on Earth, is eafily proved.
That all Motion is naturally rectilinear, is allow'd by all 5 thofe Bodies, therefore, which move in Curves, muft be re- tain'd therein by fome Power continually acting on them. Hence, the Planets being found to revolve in curvilinear Orbits, we infer that there is fome Power, by whofe con- flant Influence, they are prevented from flying offin Tan- g ents - Again
