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whereby an Agent produces an Effect ; in which fenfe, Heat is faid to be the Medium or Mean, whereby Fire acts on the Hand. Medium /«/» quo, is that which ren- ders the Power to act, compleat, in the general j without determining it to any particular Object : In this fenfe, Light is the Medium, under which the Eye perceives any Colour. ■ ■■Medium In quo, is that, by infpection whereof, a Power is produced in any things of knowing or perceiving another ; Such, is a Speculum, as it Jhews an Object 5 an Image, as it reprefenrs the Reality, £-?c
Medium, in Mechanical Philofophy, is that Space or Region, thro' which a Body paffes in its Motion towards any Point. Thus JEther is fuppofed to be the Medium , wherein the heavenly Bodies move. Sec jEther, stir thcMed'wm wherewith the Earth is encompafs'd, and Ear- thy Bodies move. See Air. Water the Medium in which Fifties live and move. See Water. Thus, Glafs is alfo a Medium of Light, as it affords it a free Paffage : And that Denfity or Confidence in the Parts of the Medium, whereby the Motion of Bodies in it is retarded, is call'd the Re- fiftance of the Medium 5 which, together with the Force of Gravity, is the Caufe of the Ceflation of Motion of Pro- jectiles. See Resistance of the Medium.
Subtile Medium. Sir If. Newton makes it probable, That befide the particular Aerial Medium, wherein we live and breath, there is another more univerfal one, which he calls an JEtherial Medium j vaftly more rare, fubtile, e- laftic, and active, than Air j and by that means freely per- meating the Pores and Interlaces of all other Mediums, and diffufing itfelf thro* the whole Creation : And by the Intervention hereof he thinks it is, that moft of the great Phenomena of Nature are effected. This he feems to have recourfe to, as the firft and moll remote Phyfical Spring ; and the Ultimate of all natural Caufes. By the Vibra- tions of this Medium, he takes Heat to be propagated from lucid Bodies '? and the Intenfenefs of Heat increas'd and preferv'd in hot Bodies, and from them communicated to cold ones. See Heat. By this Medium he takes Light to be reflected, inflected, refracted, and put alternately in Fits of eafy Reflection and Tranfmiffion 5 which Ef- fects he el fe where afcribes to the Power of Attraction : fo that this Medium appears the Source and Caufe of At- traction. See Light, Reflection, Refraction, Inflection, and Attraction.
Again, this Medium being much rarer within the hea- venly Bodies, than in the heavenly Spaces 5 and growing denfer, as it recedes further from them : he propofes as the Caufe of the Gravitation of thefe Bodies towards each other, and of the Parts towards the Bodies. See Gra- vitation.
Again, from the Vibrations of this fame Medium, excited in the bottom of the Eye by the Rays of Light, and thence propagated thro the Capillaments of the Optic Nerves into the Senfory, he takes Vifion to be performed 5 fee Vision : And Co Hearing, from the Vibrations of thisor fome other Medium, excited in the Auditory Nerves, by the Tremors of the Air, and propagated thro the Ca- pillaments of thofe Nerves into the Senfory : and thus of the other Senfes. See Sensation, Hearing, £^c.
And, again, he conceives Mufcular Motion to be per- form'd by the Vibrations of the fame Medium, excited in the Brain at the Command of the Will, and thence pro- pagated thro the Capillaments of the Nerves into the Mufcles ; and thus contracting and dilating them. See Muscle.
The Elaftic Force of this Medium, he mews, muft be prodigious: Light moves at the rate of 70,000,000 Miles in about feven Minutes, yet the Vibrations and Pulfes of this Medium, to caufe the Fits of eafy Reflection and cafy Tranfmiffion, mutt be fwifter than Light, which yet is 7co,oco times fwifter than Sound. The Elaftic Force of this Medium, therefore, in proportion to its Denfity, mufl be above 490,000,000,000 timesgreater than the Elaftic Force of the Air, in proportion to its Denfity : The Velocities and Pulfes of Elaftic Mediums being in a fub-duplicate Ra- tio of the Elasticities, and the Rarities of the Medium, ta- ken together. And thus may the Vibrations of this Me- dium be conceiv'd as the Caufe of Elasticity of Bodies. See Elasticity.
Further, the Particles of this Medium being fuppofed infinitely fmal], even f mailer than thofe of Light ; if they be likewife fuppofed, like our Air, to have a repelling Power, whereby they recede from each other, the Small- nefs of the Particles may exceedingly contribute to the Increafe of the repelling Power, and confequently to that of the Elafticity and Rarity of the Medium ; and fo fit it for the free Tranfmiffion of Light, and the free Motions of the heavenly Bodies. In this Medium may the Planets and Comets roll without any confiderable Refiftance. If it be 700,000 times more eiaftic, and as many times rarer, than Air; its Refiftance will be above 600,000,000 times lefs than that of Water ; A Refiftance that would
make no fallible Alteration in the Motion of the Planets in ten thoufand lears. And is not fuch a Medium better difpofed tor the heavenly Motions, than that of the Car- tejians, which fills all Space adequately, and without leav- ing Pores ■■, is valtly denfer than Gold •■, and therefore mult refill: more ? See Plenu m, l$c. If any ask how a Me- dium can be fo rare, let him tell how the Air, in the upper Regions ot the Atmofphere, can be above a hundred thoufand times rarer than Gold. How an electric Body can, by Friction, emit an Exhalation fo rare and fubtile yet fo potent, -as, tho its Emiffion occafions no fenfible Alteration in the Weight of the Body j yet it /hall be diffufed thro a Sphere of two Foot in Diameter, and carry up Leaf-Copper, or Leaf-Gold, at the diftance of a Foot from the electric Body. Or how the Effluvia of a Mag- net can be lo fubtile, as to pafs a Plate of Glafs without any Refiftance or Diminution of Force j yet fo potent, as to turn a Magnetic Needle beyond the Glafs. See Ei.ec-
TR ICJ TY.
That the Heavens are not fill'd with any other, but fuch a fubtile ethereal Medium, is evident from Phenome- na 5 whence elfe thofe lafting and regular Motions of the Planets and Comets, in all manner of Courfes and Direc- tions. And how are luch Motions confident with that Refiftance, that muft refult from that denfe, fluid Medium- wherewith the Cartejians fill the Heavens. The Re- finance of fluid Mediums arifes partly from the Cohefion of the Parts of the Medium, and partly from the Vis Xner? ti*. The firft, in a fpherical Body, is nearly as the Dia- meter, or, at molt, as the Factum of the Diameter, and the Velocity of the Body. The latter is as the Square of that Factum. Thus are the two Kinds of Refiftance di- ftinguifh'd in any Medium 5 and being diftinguifh'd, it will be found that almoft all the Refinance of Bodies, moving in ordinary Fluids, arifes from the Vis Inertia. That Part which arifes from the Tenacity of the Medium, may be diminiJh'd, by dividing the Matter into fmaller Parts, and making thofe more fmooth and flippery : But the other will ftill be proportional to the Denfity of the Matter, and cannot be dimini/h'd any other way, but by a Diminution of the fame. Thus the Refiftance of fluid Medium^ is nearly proportional to their Denfities 5 and thus the Air we breath, being about oco times lighter than Water, muftrefift about poo times lefs than Water: As, in effect, the lame Author has found it does by Expe- riments on Pendulums. Bodies moving in Quick-Silver, Water, or Air, don't appear to meet with any other Re- fiftance, but what arifes from the Denfity and Tenacity of thofe Fluids 5 which they muft, were their Pores fill'd with a denfe and fubtile Fluid. See Vacuum. Heat, 'tis found, diminishes the Tenacity of Bodies very much 3 yet does it not decreafe the Refiftance of Water, fenfibly. The Refiftance of Water, therefore, arifes chiefly from its Vis Inertire--, confequently, if the Heavens were as denfe as Water, or as Quick-Silver, they would not refill much lefs: if abfolutely denfe, without any Vacuum, be the Particles never fo fubtile and fluid, they would refill much more than Quick-Silver. A folid Globe, in fuch a Medium, would lofe above half its Motion, while it moves thrice the Length of its own Diameter ; and a Globe not perfectly folid, fuch as the Planets, would lofe more. To make way therefore for the lafting Motions of the Planets and Comets, the Heavens muft be empty of all Matter, except, perhaps, from fome very fine Effluvia 5 from the Atmofpheres of the Earth, Planets, and Comets 5 and fome fuch jEtherial Medium as we have defcribed. A denfe Fluid can ferve for no purpofe, in the Heavens, but to dillurb the Celeftial Motions, and to make the Frame of Nature languifti; and in the Pores of Bodies, ferves only to check the vibrating Motion of their Parts, wherein their Heat and Activity confifts. Such a Medium, there- fore, unlefs we had fome Evidence of its Exiftcnce, muft be given up ; and that given up, the Hypothcfis of Light's confifting in a Predion, falls to the ground. See Light^
PrESSION, CaRTESI ANISM, &C.
MED1US Venter, in Anatomy, the Brcaft or Thorax. See Thorax j fee alfo Venter.
MEDULLA, in natural Hiftory, &c. See Marrow.
Medulla Cerebri and Cerebelli, is the white foft Part of the Brain ; and Cerebellum, cover'd on the Outfide with the Cortical Subftance, which is of a more dark or afhy Colour. See the Origin, Structure and Ufe thereof, under Brain and Cerebellum.
MEDULLA Oblongata, is the Medullary Part of the Brain and Cerebellum join'd in one; the fore-part of it com- ing from the Brain, and the hind-part from the Cerebel- lum. It lies on the Bafis of the Skull, and is continu'd thro' the great Perforation thereof into the Hollow of the Vertebra: of the Neck, Back, and Loins: tho' only fo much of it retains the Name, as is included within the Skull. After its Exit thence, it is diftinguifiYd by the Name of the Medulla Spinalis,
The.
