Page:Cyclopaedia, Chambers - Volume 2.djvu/168

 MEG

( ?2I )

MEC

Circle in a Mathematical Point; which has no Parts or Dimenfions, ccnfcquently no Magnitude : but a Thing that has no Magnitude or Dimenfions, bears no proportion to another that has ; and cannot therefore mcafure it. Hence we fee the Reafon of the Division of Circles into jtfo Parts or Arches, called Degrees. See Degree.

Measuring of Triangles, or from three given Sides or Angles, to determine all the reft, is call'd Trigonometry. See Trigonometry.

Measuring of tie Air; its PrefTure, Spring, £?c. is called Aerometry or Pneumatics: See Aerometry, i£c.

MEATUS Cyfticus, a Bilary Duct, about the Bignefs of a Goofe-Quill, which at about two Inches diftance from the Gail-Bladder, is join'd to the Meatus Hepaticus ; and thefe together form the DuHus Communis. See Bile.

Meatus Urinaria*, or Urinary Pajjage, in Women, is very fhorr, lined internally with a very thin Membrane ; next to which is a Coat of a white Subftance. Thro this Coat, from forne Lacunx in it, pafs feveral Ducts, which convey a limpid glutinous Matter, ferving to anoint the Extremity of the Urethra. See Urinary.

Meatus Auditorial, the Entrance of the Ear ; a carti- laginous Subftance, irregularly divided with fle/liy mem- branous Interpolations in feveral Parts of ir, not unlike the Bronchia in the Lungs, only its fle/hy Fibres are here thicker. The inner Part, or that next the Brain, is bony. It is lined throughout with a thin Membrane, derived from the Skin, which is continued on the MembranaTym- pani, where ir becomes thinner. See Ear.

From the beginning of the Meatus, almoft half-way, a- rife a great number of fmall Hairs, at whofe Roots iffue the Ear- Wax, which is intangled in thofe Hairs, the bet- ter to break the Impetus of the external Air, and prevent its too fuddenly ruining in on the MemhranaTympani. See Cerumen.

MECHANICS, from ««x«"\ Engine, is a mix'd Ma- thematical Science, which confiders Motion, its Nature and Laws, with the Effects thereof, in Machines, S?c. See Motion.

That part of Mechanics which confiders rhe Motion of Bodies arifing from Gravity, is by fome call'd Statics. See Gravity, Statics, Rehistencb, g?c. In diftinflion from that part which confiders the Mechanic Powers, and their Application, properly call'd Mechanics. See Me- chanic Powers, Machine, Engine, Friction, Eq.uh.i-

BRIVM, iSc.

Mechanic Powers, are the five fimple Machines ; to which all others how complex foever, are reducible, and of the Affembiage whereof they are all compounded. See Power and Machine.

Thefe Mechanic Powers (as they are call'd) are fix, viz. the baltance, Lever, Wheel, Pul'y, Wedge, and Screw ; which fee under their proper Heads : Ballance, Lever, £/c.

They may, however, be all reduced to one, viz-, the Lever. The Principle whereon they depend, is the fame in all, and may be conceived from what follows.

The Momentum, Impetus, or Quantity of Motion of any Body, is the FaBttm of its Velocity, (or the Space it moves in a given Time, fee Motion) multiplied into its Mafs. Hence it follows, thar two unequal Bodies will have equal Moments, if the Lines they defcribe be in a reciprocal Ratio of their Maffes. Thus, if two Bodies, fallen 'd to the Extremities of a Ballance or Lever, be in a reciprocal Ratio of their Dillanccs from the fixed Point ; when they move, the Lines they defcribe will be in a reciprocal Ra- tio of their Maffcs. Kg. If the Body A {Tab. Mecha- nics, fig.6.) be triple the Body B, and each of them be fo fix'd to the Extremities of a Lever AB, whofe Ful- crum, or fix'd Point, is C, as that the Diftance ofBC be triple the Dillance C A ; the Lever cannot be inclined on either fide : but that the Space BE, pafs'd over by the lefs Body, will be triple the Space AD, pafs'd over by the great one. So that their Motions or Moments will be equal, and the two Bodies in Equilibrio. See Motion. Hence that noble Challenge of Archimedes, datis Viribus, da- tum Pondv.s movere ; for as the Diftance C B may be in- creafed infinitely, the Power or Moment of A may be in- crcafed infinitely. So that the whole of Mechanics is re- duced to the following Problem.

Any I'ody, as A, with its Velocity C, and alfo any other Body, as B, being given ; to find the Velocity neceffary to make the Mome-.-t, or Quantity of Motion in B, equal to the Moment of A, the given Body. Since, now the Moment of any Body is equal to the Rectangle under the Velocity, and the Quan- tity of Matter ; as B : A : : C : to a fourth Term, which will be c, the Celerity proper to B, to make its Moment eq ial to that of A. Wherefore in any Machine or Engine, if the Velocity of the Power be made to the Velocity of the Weight : : reciprocally as the Weight is to the Power ; then fhall the Power always fuflain, or if the Power be a .little increas'd, move the Weight.

Let.forinftance, A B be a Lever, whofe Fulcrum is ate, and let it be moved into the Pofition a c b. Here the Velo- city ot any Point in the Lever, is as the Diftance from the Centre For let the Point A defcribe theAnh A», and the Point B the Arch Bb ; men thefe Arches will be the Spa- ces dekribed by the two Motions : but fince the Motions are both made m the fame time, the Spaces will be as the Velocities. But it is plain, the Arches A a and Bb will be to one another, as their Radii A C and A B, becaufe the Sectors A C«, a 8 d Bci, are iimilar: wherefore the Velocities of the Points A and B, arc as their Diftances from the Centre C. Now ,f any Powers are applied to the Ends or the Lever A and B, in order to raife its Arms up and down; their Force will be expounded by the Perpendi- culars S a, and b N ; which being as the right Sines of the former Arches, b B and a A, will be to one another alfo as the Radii Ac, and c B 5 wherefore the Velocities of the Powers, are alfo as their Diftances from the Centre. And fince the Moment of any Body is as its Weight, or gravitating Force, and its Velocity conjunctly ; if diffe- rent Powers or Weights are applied to the Lever their Moments will always be as the Weights and their Diftances from the Centre conjunctly. Wherefore if to the fame Lever, thete be two Powets or Weights ap- ply'd reciprocally, proportional to their Diftances from the Centre, their Moments will be equal ; and if they aft contrarily, as in the Cafe of a Stilliard, the Lever will remain in an horizontal Pofition, or the Ballance will be in Equilibrio. And thus it is eafy to conceive how the Weight of one Pound may be made to equi-ballance a thoufand, gfc. Hence alfo it is plain, that the Force of the Power is not at all increafed by Engines; only the Ve locity of the Weight in either lifting or drawing, is f di- mimin d by the Application of the Inftrument, as that the Moment of the Weight is not greater than the Force of the Power. Thus, for inftance ; if any Force can elevate a Pound Weight with a given Velocity, it is impoflible by any Engine to effect, that the fame Power (hall raife two Pound Weight, with the fame Velocity : But by an En- gine it may be made to raife two Pound Weight, with half the Velocity; or icooo times the Weight with _i__ f the former Velocity. See Perpetual Motion.'""

Mechanical Curve, a Term ufed by Ties Cartes for thofe Curves, which cannot be defined by any Equation ; in oppofition to Algebraic, which they call Geometric Curves. Thefe Curves, Sir If. Newton, M. Leibnitz, Sec. call tranfeendent Curves ; and diffent from Cartes, in ex- cluding them out of Geometry. Leibnitz has even found a new kind of tranfeendent Equations, whereby thefe Curves are defined: They are of an indefinite nature; that is, don't continue conftantly the fame in all Points of the Curve ; in oppofition to Algebraic Equations, which do. See Curve.

Mechanical AffeBions, are fuch Properties in Matter as refult from their Figure, Bulk, and Motion : Mecha- nical Caufes ate thofe founded on fuch Affections; and Mechanical Solutions are Accounts of Things on the fame Principles.

Mechanical Fhilofophy, is the fame with the Corpuf- cular Philofophy ; viz. that which explains the Effects of Nature, and the Operations of Corporeal Things, on the Principles of Mechanics ; the Figure, Arrangement, Difpo- fition, Motion, Greatnefs or Smallnefs of the Parts which compofe natural Bodies. See Corpuscular.

The Term Mechanical is alfo applied to a kind of Reafoning, which of late has got a great deal of ground both in Phyfics and Medicine ; fo call'd, as being conform- able to what is ufod in the Contrivance, and accounting for the Properties and Opetations of Machines. This feems to have been the Refult of rightly fludying the Powers of a human Mind, and the Ways by which it is only fitted to get acquaintance with material Beings : For confider- ing an Animal Body as a Compofition out of the fame Matter, from which all other material Beings are formed and to have all thofe Properties, which concern a Phy- fician's Regard only, by virtue of its peculiar Make and Conftrucr ure ; it naturally leads a Perfon, who trufts to proper Evidences, to confider the feveral Parts, according to their Figures, Contexture, and Ufe ; either as Wheels' Pullies, Wedges, Levers, Skrews, Chords, Canals, Ci- tterns, Strainers, and the like ; and throughout the whole of fuch Enquiries, to keep the Mind clofe in view of the Figures, Magnitudes, and mechanical Powers of every Part or Movement ; juft in the famemannei, as is ufed, to en- quire into the Motions and Properties of any other Ma- chine. For which purpofe it is frequently found helpful to decypher, or picture out in Diagtams, whatfoever is under conlideration, as it is cuftomary in common Geo- metrical Demonftrations ; and the Knowledge obtained by this Procedure, is called Mechanical Knowledge.

Rn

Th«