MOT
Laftly, Relatively f roper Motion is the fucceflive Appli- cation ot a Body to the diftercnr pans of the contiguous Bodies. And thisis the Motion ufually underftood in'phy- ficai Difputes about the Natures of particular things ; as when we fay, that Heat, Sound, Fluidity, &c, confnt in Motion. "
( W)
MOT
A,frl r " al hand5 ' that M " im !s "oeGTentiai
Attribute of Matter; hence arifes a difpute about its Fro- StBion, and to what Caufe it owes its Qmtinmtim.
i he Cartefims maintain, that the Creator at the Ee a in- ning impreisda certain Onm.lrv nfu.,; b-j:„.
,,.,•>• , "•-"' °ounu, riuiuny, ssc, connit in wng mipreis d a certain (;,,„ri,v „f v , « ?
.on. This mult be added however, that by fucceflive and that under fuch Laws as 1? nn t ■"» ^S? 5
plication ot a Body, its whole Surface, taken together, loll, but the fame PorX'n „ A' , T ' "' ',' ^ b&
Appl
mu
contiguous Bodies.
from thefe feveral Definitions of Motion arife as many Definitions of Place: For when we fpeak of fVofioH (or Kelt; truly and absolutely proper, then Place is that pare
Body llrike on any other ¥ody77b7form," htLTZorl its Motion than it communicates to the latter.
TESIAN. '• SeeCi *-
of infinite and immoveable Space which the Boiy' fik ^t^^^^^^^ ^1°^ When amotion relative!* common, then Place is a part of ' JMfcft*,, it IS manXt hereTn atwlTtfe °n W ° any Space or moveable Dimenfio, When ot wJnrela- < ticy of J**, fa "he W^dV^rZ tlh^""
J gethcr by a fender Wire, revolve with an uniform Mrj
ttvefy proper t (which is really very improper) Place is then the burtace of the next adjacent Bodies, or fenfible Spaces. See Place.
The Definition of Reft is pretty well agreed on 5 but whether Reft be a mere Privation of Motion, or any thing poiitive, is hotly difpsred. MaUbranch, and others, main- tain the former fide of the Queftion. Cartes, and others, the latter: Thefe laft contend, that a Body at reft, has no power to perfevere in that reft, nor to refill any Bodies that would defir»y it ; and that Motion may as well be call'd a Ceflation of Kelt, as Reft of Motion. See Rest.
The main Argument urg'd by the former is this : Sup- pofe a Globe at reft, and fuppofe God ceafe to will its reft 5 What will be the Conference ? it will reft (till. Let it be in Motion, and let God ceafe to will its Motion ; What will be the Confequence ? it will ceafe to move 5 that is, be at reft ; becaute the Power whereby a Body in 'Motion perfeveres in that iiate, is the poiitive Wilt of God $ that whereby the quiefcent Body perfeveres, only his pri- vative Will.
But this is a manifeft Petitio principii 5 for the Force or Conatus whereby Bodies, whether moving or quiefcent, perfevere in thofe ftates, is the mere Inactivity of Matter ; and ■ therefore were it poflible for God not to will any thing, a Body already in Mot'mt, would move for ever ; as a Body at Reft, would reft for ever. To this Inactivity of Matter, it is owing that all Bodies rcfift according to
appears, rea-
about their common Centre of Gravity, and at the fame
_ time that Centre be carried uniformly in a right Line drawn
,'" the lla ne of their circular Afa»; 2 ; the Sum of the M*-
Hon ot the two Balls, as often as they ate in a right Line
_ drawn from tneir common Center of Gravity, will be
greater than the Suiu of their Motion when they are in a
Line perpendicular to that other. Whence it appei
( that Mono,: may both be generated and loft. But by t
ion of the tenacity of fluid Bodies, and the lift, on of
their Parts, with the Wcaknefs of the elaftic Power in folid
Bodies, Mature feems to incline much rather to the De-
' ftruflion than the Produflion af-Motion ; and in reality
' Motion becomes continually lefs and lefs. For Bodies
' which are either fo perfectly hard, or fo foft, as ro have
no elaftic Power, will not rebound from each other :
' ^. he "' Impenetrability will only Hop their Motion. And
• it twoluch Bodies, equal to each other, 1
' equal but oppolite Motions, fo
, . J as ro n ' by the Laws at Motion they mull
be carried with
el in a void Space,
Hop in the very Place
'of Concourfe, lofeall their Motion, and be at reft fo, | ever, unlefs they have an claflic Power to give 'em a
new Motion. 11 they have Elalticiry enough to enable em to rebound with 4, or J, or J of the Force where- with they-meet, they will iofe |, or 4, or j of their Mo. tion. And this is confirmed by Experiments- For if , s . ' two equal Pendulums be let fall from equal heights fo
their Quantities of Matter, and that any Body finking ' as to firikefull on each other 5 if thofe Pendulums be of
' Lead, or foft Clay, they wililofe all, bralmoit all their Motion ; and if they be of any Elaftic Matter, they will
another with any given Velocity, will move it in the fame
Proportion winch its Denfity, or Quantity of Matter has,
to the Deniity or Quantity of Matter of the other.
iVJotiow nas ever been efteem'd a Species cf Quantity ;
and its Quantum, or 'Greatnefs, which we otherwife cat] its
Momentum, is effimated partly from the length of the Line
which the moving Body defct ibes ; as if a Body pals over a
Line of 100 Feet, the Quantity of its Motion is greater
than if it patTed over 10 Feet : and partly from the Quan- tity of the Matter moved together, or at the lame time
i. c. not from the Bulk or Extenfion of the Body, but
from its Mais or Weight ; the Air, and other fubrile Mat- ters wherewith the Pores are filled, not entring m,o the
account : As if a Body of two cubic Feet run over a Line of 100 Feet, the Quantity of its Motion is greater than that of a Body of one cubic Foot defcribing the fame Line : For whatever Motion one Whole has, that is had in one Half of the other ; and the Motion of the Whole is the Sum of the Motion of all its Parts.
Hence it follows, that for two unequal Bodies to have equal Motions, or Momentums, the Lines which they pafs over mull be in a reciprocal Proportion of their Mafles or
Weights, i.e. if one Body have three times the Quantity tremely controverted among PhyficalWriters,and yet follows of Matter that another has, the Line it runs over muft be very evidently from one of'the grand Laws of Nature viz, 1 ot the Line run over by the other. If two Bodies then That all Bodies perfevere in their prefent State wh'erh-r taftencd to the two Extremities of a Balance or Lever, of Reft ot Motion, unlefs diftutbed by fome foreign Powers.
Motion therefore once begun, would be continued in infin. '
' only retain fo much Motion as they receive from their 'elaftic Power.' If it be asked, how it happens that Mo- tins being thus continually loft, fhould be continually re- newed : The fame Author adds, that it is renewed from fome aflive Principles, ' Such as the Cattfe of Gravity ' whereby the Planets and Comets preferve their Motwnsin theit Orbits, and all Bodies acquire a great degree of ' Motion in falling ; and the Caul'e of Fermentation, where- ' by the Heart and Blood of Animals preferve a perpetual ' Warmth and Motion ; the inner parts of the Earth are ' kept continually watm'd ; many Bodies burn and Urine ; 'and the Sun himfelf burns and fhines, and with his Light ' warms and chears all things,' (as alj'o from the Caufe of Elajiicity, by which Bodies rejtore tbemfehes into their former Figures ;) ' For we find but little Motion in the World ' except what plainly flows, either from the aflive Prin- ' ciples, or from the Command of the Wilier.' See Gra- vity, Fermentation, Elasticity, £-?c.
As to the Continuation of Motion, or the Caufe why a Body nee in Motion comes to perfevere in it ; this has been ex-
have their rVlafles in a reciprocal Ratio of their Diftances from the fixed Point, when thefe are moved they muft de- scribe Lines in a reciprocal Ratio of thefe Mafles.
For inftance, if the Body A (Tab. Mechanics, JiV. 3c.) have three times the Mais or Weight of B, and each be fallen'd to the Extremities of the Lever A B, whofe Fulcrum or fixed Point is C, in fuch manner as that the Diftance B C is thrice the Diftance C A 5 the Lever can't
turn, were it to meet with no intetruption from external Caufes; as the Power of Gravity, the Refiftance of the Medium, t£c. So that Jriftotle's Principle, Whatever is moving affefls Rt'ft, is gtoundlefs. See Nature.
Nor has the Communication of Motion, or the manner how a moving Body comes to affefl another at Reft; or how much of its Motion is communicated by the firft to the
incline either way, but the Space which the lefs Body laft, been lefs difputed. See the Laws thereof under the moves will be thrice the Space A D, which the greater Word Percussion.
moves: So that their Motions will be equal ; nor there any reafon why the Body A tending downwards, v. %. with four Degrees of Motion, fhould raife the Body B, rather than B tending downwards likewife with the fame four Degrees of Motion, fhould raife the Body A : They
Motion, we have obferv'd, is the Subject of Mechanics; and Mechanics is the Bafts of all Natural Philofophy ; which is hence denominated Mechanical. See Mecha- nical.
In effect, all the Phenomena of Nature ; all the Chan
will therefore be in Equilibria On which Foundation de- ges that happen in the Syftem of Bodies,' are owing to
pends the whole Doflrine of Mechanics. Motion ; and are direfled according to the Laws thereof.
Hence that great Problem of Archimedes, Aot tn s5 *J tiv Hence, the modern Philofophers have apply'd them-
ylji tufitm ; with any given Power, how fmall foever, to felves particularly to cultivate the Doflrine of Motion ; to
raife any Weight given, how great foever ; for by increa- inveftigate the Properties, Laws, (gc. thereof; by Obfer-
fing the Diftance C B infinitely, the Power of the Body A vation,' Experiment, and the Ufe of Geometry. And to
will be increafed infinitely. See Mechanics and Eojii- this we owe the great Advantages of the modern Philofo-
lihrium. phy above that of the Antients ; who were extremely dif-
regardfui
