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This Hypothecs, tho' ingenious enough, yet, as it is founded on the Suppofition of a Plenum, it is overthrown by what we fliall prove of the Exiftence of a Vacuum. Dr. Hook inclines to an Opinion much like thatofZta Cartes: Gravity, he thinks, deducible from the Action of a moftfubtle Medium,which eafily pervades, and penetrates the moft folid Bodies; and which by fome Motion it has, detrudes all earthly Bodies from it, toward the Centre of the Earth.
Vofjius, and many others give partly into the Canejzan No- tion, and fuppofe Gravity to arile from the Diurnal Rota- tion of the Earth round its Axis.
Dr. Halley, defpairing of any fatisfacfory Theory, chufes to have immediate Recourfe to the Agencvof the Almighty. So, Dr. Clark, from a. View of feveral Properties of Gra- vity, concludes, that it is no adventitious Effect of any Mo- tion, or fubtle Matter, but an Original, and General Law imprefs'd by the Almighty on all Matter ( and preferv'd. therein by fome efficient Power penetrating the very folid, and intimate Subftance thereof; as being found always pro- portionable, not to the Surfaces of Eodics or Corpulcles, but to their folid Quantity and Contents.
It fhould, thererorc, lays he, be no more enquired, why Bodies gravitate, than how they came to be firit put in Mo- tion. — Annot. in Rohaulti c Phyf. P. c. 28.
Dr. S. Gravcfande, in his IntroduB. ad Philofoph. Newton* contends, that ihe Caufe of Gravity is utterly unknown ; and that we are to look on ir no otherwile than as a Law of Nature, originally and immediately impreffed by the Cre- ator, without any Dependance on any fecond Law or Caufe at all. Of this he thinks, the three following Considera- tions, fufflcient Proof
ijl. That Gravity requires the Pretence of the gravitating or attracting Body : So the Satellites of Jupiter, e. gr. gra- vitate towards Jupiter wherever he be.
2d. That the Diftance being fuppofed the fame, the Ve- locity wherewith Bodies are moved, by the Force of Gravity, depends on the Quantity of Matter in the attracting Body : And that the Velocity is not changed, let the Mafs of the gravitating Body be what it will.
Lafily, That {{Gravity do depend on any known Law of Motion, it muft be fome Impulfe from an Extraneous Body; whence, as Gravity is continual, a continual Stroke wou'd be required.
Now, if there be any fuch Matter continually {Inking on Bodies, it mull be fluid, and fubtle enough to penetrate the Subftance of all Bodies: But how fhall a Body fubtle enough to penetrate the Subftance of the hardeft Bodies, and io rare as not (enfibly to hinder the Motion of Bodies, be able to impel vaft Bodies towards each other with fuch Force ? How does this Force increafe in a Ratio of the Mafs of the Body toward which the other Body is moved ? whence is it that all Bodies, fuppofing the fame Diftance, and the fame Body gravitated to, move with the fame Velo- city ? Can a Fluid, which only acts on the Surface, either of the Bodies themfelves, or of their internal Particles, com- municate fuch a Quantity of Motion to Bodies, which in all Bodies fhall exactly follow the proportion of the Quantity of Matter in them ?
Mr. Cotes, goes yet further ; giving a View of Sir If. New- ton's Philofophy, he affeits that Gravity is to be rank'd a- mong the primary Qualitiesof all Bodies; and deem'd as ef- fential to Matter, as Extenfion, Mobility, or Impenetrabi- lity. 'Prtffat. ad Newton. Trincip.
But this Author teems to have overfhot the Mark. His great Mailer, Sir I.Newton himfelf, difclaims the Notion; and to fhew that he docs not take Gravity to be effential to Bodies, gives us his Opinion about the Caufe 5 chufing to propofe it by way of Query, as not being yet fufficiently iatisfied about it by Experiments.
This Query we fliall fubjoin at large— After having JViewn that there is a Medium in Nature vaftly more fub- tle than Air, by whofe Vibrations Light communicates Heat to Bodies, and is itfelf put into alternate Fits of eafy Reflexion and eafy Tranfmiflion, and Sound propagated • and by the different Densities whereof the Refraction and Reflexion of Light is perform'd. See Medium, Heat, Refraction, ckc. He proceeds to enquire,
« Is not this Medium much rarer within the denfe Bodies
- of the Sun, Stars, Planets, and Comets, than in the empty
- celeftial Spaces between them? And in paffing from them
4 to great Distances, doth it not grow denfer and denfer per-
- petually, and thereby caufe the Gravity of thofe great Bo-
- dies towards one another, and of their Parts towards the
- Bodies : every Body endeavouring to go from the denfer
- Parts of the Medium towards the rarer ?
- For if this Medium be fuppofed rarer within the Sun's
- Body than at its Surface, and rarer there than at the hun-
- dreth Part of an Inch from its Body, and rarer there than
' at the fiftieth Part of an Inch from its Body, and rarer 1 there than at the Orb of Saturn; I fee no Reafbn why
- the Increafe of Denfity fhould flop any where, and not
- rather be continued thro' all Diftances from the Sun to Sa-
- turn, and beyond.
' And tho' this Increafe of Denfity may at great Diftances be exceeding How, yet if the Elaftic Force of this Me- dium be exceeding great, it may fuffice to impel Bodies from the denfer Parrs of the Medium towards the rarer with all that Power which we call Gravity. f And that the Elaftick Force of this Medium is exceed- ing great, may be gather'd from the Swiftnefsof its Vibra- tions. Sounds move about 1140 Engliflo Feet in a fecond Minute of Time, and in feven or eight Minutes of Tims they move about one hundred Er.glifJo Miles : Light moves from the Sun to us in about feven, or eight Minutes of Time, which Diftance is about 70000000 Englifh Miles, fuppofing the Horizontal Parallax of the Sun to be about 12. ;and theVibrations or Pulfesof this Medium, that they may caufe the alternate Fits of eafy Tranfmiffion, and eafy Reflection, mult be Iwifter than Light, and by confequence, above 70-0000 fwifter than Sounds; and therefore the Ela- ftick Force of this Medium, in Proportion to its Denfity, muft be above 7G0000 x 700000 (that is above 490000000000) times greater than the Elaftick Force of the Air, is in Pro- portion to its Denfity : For the Velocities of the Pulfes of Elaftick Mediums, are in a fubduplicate Ratio of the Ela- fticities and the Rarities of the Mediums taken together.
- As Magnetifm is ftronger in fmall Loadftones than in
great ones, in proportion to their Bulk ; and Gravity is ftronger in the Surface of fmall Planets, than in thofe of great ones, in proportion to their Bulk ; and fmall Bodies are agitated much more by Electric Attraction, than great ones : So the Smallnefs of the Rays of Light, may contri- bute very much to the Power of the Agent by which they are refracted, And if any one mould fuppofe that iEther (like our Air) may contain Particles which endeavour to recede from one another(for Ido notknow what thisiEther is) and that its Particles are exceedingly fmaller than thole of Air, or even than thofe of Light; the exceeding Small- nefs of fuch Particles may contribute to the Greatnefs of the Force by which they recede from one another, and thereby make that Medium exceedingly more rare and elaftick than Air, and of confequence exceeding- ly Icfs able to refilt the Motions of Projectiles, and exceed- ingly more able to prefs upon grofs Bodies, by endeavour- ing to expend it faff. Opiicks, p. 325,^. See Light, Elasticity,^. Gravity in Mechamcks, is the Conatus, or Tendency
of Bodies toward the Centre of the Earth. See Centre. That part of Mechanicks which confiders the Motion of
Bodies arifing from Gravity, is peculiarly called Staticks.
See Staticks.
Gravity is diftinguiftVd into Abfolute and Relative. Abfolute Gravity, is that wherewith a Body defcends
freely thro* another refilling Medium. See Resistance. 'The Laws of Abfolute Gravity, fee under the Article
Descent of Bodies.
Relative Gravity, is that wherewith a Body defcends
after having fpent part of its Weight in overcoming fome
Refiflance, See Resistance.
Such is that wherewith a Body defcends along an inclined
Plane, where fome part is employed in overcoming the Re-
fiftance, or Friction of the Plane. See Friction. The Laws of Relative Gravity ; fee under the Article
Inclined Plane.
Centre of Gravity, ^Diameter of Gravity 'Plane of Gravity,
" Centre of Gravity, y Diameter of Gravity, -Plane of Gravity.
Gravity in Hydroflaticks—thc Laws of Bodies gravi- tating in Fluids, make the Bufinefs of Hydroftaticks. See Hydrostaticks.
Gravity is here divided into Abfolute and Specific.
Abfolute or true Gravity, is the whole Force where- with the Body tends downward. See Gravitation.
Specific Gravity, call'd alfo Relative, Comparative, and Apparent Gravity, is the Excefs of Gravity in any Body, above that of an equal Quantity or Bulk of ano- ther.
For the Laws of Specific Gravity: with the Methods of determining it both in Solids and Fluids. See Speci- fic Gravity, and Hydrostatical Balance.
Gravity of the Air. See Weight of the Air. See alio Air and Pressure.
Gravity, in Mufic, an Affection of Sound, whereby it becomes denominated grave, low, or fiat. See Sound.
Gravity ftands in Oppofition to Acutenefs, which is that Affection of Sound, whereby it is denominated Acute, Sharp, or high. See Acuteness.
The Relation of Gravity, and Acutenefs, is the prin- cipal thing concerned in Mufic ; the Diftinanefs, and Deter- minatenefs of which Relation, gives Sound the Denomina- tion of Harmonical, or Mufical. Sec Music and Har- mony.
The
