IN (381 J IN
Light of other Colours, the Sines have Meed other Pro- portions; but the^ Difference is fo little, that it need fel- dom be confider'd. As for example ; Let r s (in the Fi- gure) be the Surface of the ftill Water, c the Point of Inci- dence, in which any Ray coming in the Ait from a in thcLine a c, is reflected or refrafled : To know whither this Ray ftiall go after fuch Reflexion, or Refraction ; on the Sur-
face of the Water rs, and in the Point c, erect the Perpen- dicular cp, and produce it downwards to q. Knowing therefore that the Ray after Reflexion or Refraction will be found fomewhere in the Plane of the jingle of Inci- dence a c f produced ; let fall the Sine of that Angle (viz. ad) on the Perpendicular f c ; and then if the Re- flected Ray be fought, produce ad to b, fo that d £=: ad, and draw cb, which ftiall be the reflected Ray, becaufe the Angle of Reflexion, and its Sine, are equal to the Angle and Sine of Incidence, as they ought to be. But if the refracted Ray be fought, produce ad to i; fo that d h may be to a d, as the Sine of the Refraction, to that of Incidence: that is, as 3 to 4. T hen with the Ra- dius a c defcribing the Circle a be, and in the Plane acp', draw the Line be parallel to pq, and cutting the Circum- ference in e: Then drawing c e, That /hall be the refrac- ted Ray. For i e/be let fall perpendicularly on the Line f 5, it ftiall be the Sine of the Angle of Refraction of the Ray c e ; and this Sine is equal to d h, and confe- quently in proportion to the Sine of the Angle of Incidence ad, as 3 to 4.
Incidence Point, in Opficks, is that Point in which a Ray of Light is fuppofed to fall on a Piece of Glafs.
INCIDENT, an Event or particular Circumflance. Incident, in a Poem, is an Epifodc or particular Action, tack'd to the principal Action, or depending on it. Thus a good Comedy is full of agreeable Incidents, which divert the Spectators, and form the Intrigue. The Poet ought always to make choice of fuch Incidents as are fufceptible of Ornaments fuitable to the Nature of his Poem. The Variety of Incidents well conducted make the Beauty of an Heroic Poem, which ought always to take in a cer- tain number of Incidents to fufpend the Catallrophe, that would otherwife break out too foon.
INCINERATION; in Chymiftry, is the Reduflion of Vegetables into Allies; which is done by burning them gently. Thus Fern is reduced into Allies for the making of Glafs. The Word is derived from the Latin Frepofi- tion in, and Cinis, Allies.
INCISIVE, in Anatomy, is underftood of feveral Teeth, a double Mufcle, and certain Orifices belonging to thofe Teeth. The mcfive Teeth, Incifores or Cutters, fome- times alfo called Rifores, becaufe they ftiow themfelves in Laughing, are eight in number, four in each Jaw, fituate in the Fore-fide, and in the middle of the othets. They are fhorter and (harper than the others, and are inferted into their Alveoli by a iingle Root or Twang ; for which reafon they fall eafily, efpecially thofe of the upper fide. They are called incijive, becaufe their Office is to cut or incide the Meat. The firft proper Mufcle of the upper Lip has alfo the Epithet of incifive, becaufe it has its Origin in the Bone of the upper Jaw, near the Place of the inc'ifivc Teeth ; hence it fades, and is inferted into the upper Lip, which it ferves to draw up. The maxillary Bones have four internal Orifices, two whereof are called incijive, as being directiv under the incifive Teeth.
INCLINATION, a Term ufed in Medicine and Chy- miftry, when a clear Liquor is poured off from fome Fie- ces, or Sediment, by only gently ftooping the Veffel ; which is alfo called Decantation.
INCLINATION, in Phyfics, expreffes the mutual Approach or Tendency of two Bodies, Lines, or Planes towards one another; fo that their Direflions make either a ftrait Line at the Point of Contaft, or an Angle of a
greater or leffer Magnitude. What the Angle of Inclini. t»» figntfies in Opticks, fee m Angle of Incidence. The Inclination of two Planes is the acute Angle made by two Lines drawn one in each Plane, and perpendicular to their common Section. Inclination of the Axis of the Earth is the Angle which it makes w,th the Angle of the Eclip- t.ck; or the Angle between the Planes of the Equator and Ecl.ptick. Inclination of a Planet is an Arch of the Circle of Inclination, comprehended between the Eclip- tick, and the Place of a Planet in his Orbit. Inclination of a Plane 111 Dialling, is the Arch of a vertical Circle perpendicular to both the Plane and the Horizon, and in- tercepted between them. To find this, take a Quadrant and apply its fide to the fide of a Square, and apply the other fide of your Square to your Plane; if the Plum- met fall parallel to the fide of the Square, then the lower fide of the Square ftands level ; by which draw an Hori- zontal Line, whereon erefl a Perpendicular, and apply your Square to that Perpendicular; and if the Plummet falls parallel to the fide of the Square, then that is alfo. a level Line, and your Plane ftands horizontally : If the Plummet falls not parallel to the fide of the Square, then turn your Square, until it does, and draw an horizontal Line, on which erect a Perpendicular, to which apply your Square, and obferve what Angle your Plummet makes on the Quadrant, with the fide of the Square ; that is, the Angle of the Inclination of the Plane. Inclination of a. Ray in Dioptricks, is the Angle which this Ray makes with the Axis of Incidence in the firft Medium, at the Point where it meets the fecond Medium. Inclination of a Right Line to the Plane is the acute Angle, which this Right Line mikts with another Right Line drawn in the 1 lane thro the Point where the inclined Line interfects it and thro the Point, where it is alfo cut by a Perpendicu- lar drawn from any Point of the inclined Lines. Inclina- tions of the Planes of the Orbits of the flanets to tha Plane of the Ecliptick, are thus. Saturn's Orbit makes an Angle of 1 Degrees 30 Minutes. "Jupiter's t Degree and f Man's little lefs than 2 Degrees. Venus's is 3 De- grees and }. Mercury's is almoft 7 Degrees.
INCLINED PLANE. Befides what has been faid under Defccnt of heavy Bodies upon inclined Planes, it may be here added : If a Body as B be partly fupported by the inclined Plane A C, and partly by the Power R afting on a Direction parallel to the Plane A C ; then that Power R : is to the Body, or Weight B : : as the Sine of the Angle of the Plane's Inclination A C B : is to the Ra- dius C A. C D, A D, G F, E D, and H F, being drawn as in the Figure ; the Body B will be fuftained, as i«
were, by three Forces, or Powers, all afling according t» different Directions, and in Equilibrio one to another., The firft of which is the Force of Gravity, expreffed by B E perpendicular to the Horizon C D. The fecond the Power R, according to the Direflion B R parallel to the Plane A C. And the third is fupplied by the Refiftance or Contranitence of the Plane AC; and is expreffed by BH perpendicular thereunto. For Action and Re-action being equal, and one acting directly contrary to the ci- ther ; the Plane which is preffed by the Body or Weight B, according to the Direction of the Perpendicular H B, muft act on that Body by a contrary Direflion, mis, that of F B or B H. And fince thefe three Powers are all mutually in Equilibrio, and that the Body B is fuftained by them, it is plain (when G F is drawn perpendicular to CD, or parallel to BE) that the Force of Gravity will be expreffed by G F ; and that the Power R : will be to the Power of Gravity : : as G B : to G F ; but in the Right angled Triangle CFG, (F B being a Perpendicular to the Bafe CG) BG : GF : : G F : GC, and as FG : G C : : A D : A C (by fimilar Triangles) wherefore the Power R : is to the Force of Gravity : : as A D : to A C, or as the Sine of the Angle of Inclination to the Radius. Wherefore the Force by which any heavy Body would defcend on any inclined Plane, to the Force of the Defcent in the Perpendicular, is as the Sine of the Angle of the E s e e « ' phne'a
