Page:Cyclopaedia, Chambers - Volume 1.djvu/170

 ARC

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ARC

See Crane,

Arbor of a Crane, z Mill, Windmill, &c, Mill, Windmill, &c.

ARBOREUS, Arboreous, is applied by fomeNatu- ralifts to fuch Excrefcencies, Fungus's, Mofles, and other Paraiites as grow on Trees. See Fungus, Mushroom, Galls, Moss, Parasite, Acorus,££c.

AREORIST, Arborista, a Perfon skill'd in Trees ; their Kinds, Forms, Natures, &C See Tree.

Arborifi is an appellation of lefs extent than Sotanifi. See Botany, fc£e.

ARBOUR,among Gardeners,^. a kind of fhady Bower or Cabinet, contrived to take the Air in. See Garden.

Arbours are now gone much into difufe, by reafon their

Seats, apt to be moinr, are unwholefcme They are

diftinguillvd "into natural and artificial

Natural arbours, are formed only of the Branches of Trees, interwoven artfully, and born up by ftrong Lattice- work, Poles, Hoops, &c. which make Galleries, Halis, Porticoes, and green Vino's naturally covered.

The Trees wherewith rhefe Arbours are formed, are u- fually the Female Elms, or Dutch Lime-Trees; in regard they eafily yield, and by their great quantity of fmall Boughs, form a thick Brulh-wood : the lower parts are filled up with Horn-beam.

Artificial Arbours, and Cabinets are made «f Lattice- work, born up by Standards, Crofs-rails, Circles and Arches of Iron. Forwhich purpofc they make ufe of fmall Fillets of Oak, which being planed and made ftrait, are wrought in Checkers, and {aliened with Wire.

ARC, or Ark. See Arch.

The Words are formed from the Latin, *fr'c$lS, a Sow.

ARCANUM literally fignifies a Secret 3 and is therefore very pertinently applied by Quacks and Impofiorsin Medi- cine, who conceal their Ignorance and Fraud under a pre- tence of Secrecy Hence Legions of Arc an a*

There are alfo flanding officinal Compofitions, under the Denomination of Arcana •• fuch are

The Arcanum Coraliinum-i a Preparation of red Preci- pitate; made by dilVilling it with Spirit of Nitre, and re- peating the Diftillation again and again, till a red Powder be procured. This Powder boiled in Water, and the Water poured off, and tartariz'd Spirit of Wine put to the Powder ; two or three Cohobations are made; which leave a Powder, much like the Prince's Powder; of good ufe in the Gout, Dropfy, Scurvy, £j?e. It operates chiefly by Stool.

sircanimzjoviale, is made of an Amaigama of Mercury and Tin, digefted in Spirit of Nitre : the Nitre being drawn off, the remaining matter is wetted with Spirit of Wine, and the Spirit burnt away 3 and this for feveral times till the pungent Tafie is gone : what remains is ufed ■much with' the fame Intentions asthc AntibeBicum c Poterii, and is recommended by fome as a Sudorific. See Anti-

HECTlCUM.

Arcanum duplication is prcpar'd of Caput Mortuum of Aquafortis, by diffolving it in hot Water, filtrating, and e- vaporating it to a Cuticle j then leaving it to fhoot.

The Arcanv.m duplication, or 'Panacea duflicata is ex- tolled as a Diuretic, and Sudorific The Recipe was pur- chafed at the expence of 500 Dollars, by that great Virtuofo theDukedf Eoljbein* Schroder, that Prince's Phyfician, writes wonders of its great ufes in Hypocondriacal C:fes, continu- ed and intermitting Fevers, Stone, Scurvy, $Sc

ARC-Boutant; in Building, a flat Arch or part of an Arch abutting againft the Reins of a Vault, to fupport, and prevent its giving Way. See Vault.

Jrc-houiants are only arch'd Buttreffes. See But-

TRESSE.

The name is French ; form'd of arc and bouter, to a-but

ARCH, Ark, Arcus, a part of any curve Line 3 e.gr. ofa Circle, Ellipiis, or the like. See Curve.

Arch of a Circle is a part of the Circumference thereof, lefs than a half, or Semicircle --Such is AB (Tab. Geo- metry, fig. 2,7) See CiRCtE and Circumferfnce.

The fcafe or Line that joins the two Extremes of the Arch is called* the Chord s and the Perpendicular rais'd in the middle of that Line, the Sine of the Arch. See Chord and Sine.

All Angles arc meafured by Arches —to know their

Quantity, an Arch is defcribed, having its Centre in the Point of the Angle. See Angle.

Every Circle "is fuppofed to be divided into 3<5o Degrees 3 and an Arch isefiimated according to the Number of thofe

Degrees it takes up Thus an Arch is faid to be of

30, of 80, of 100 Degrees. See Degree. Hence

Equal Arches are fuch Arches of the fame or equal Circles, as contain the fame Number of Degrees. See Eq_ual— — Hence

In the fame or equal Circles, equal Chords fubtend equal

Arches And hence, again, Arches intercepted between

parallel Chords arc equal.

A Radius, CE, fig.98. which biffecls the Chord in D ctoe5 alfo bifleft theArch, in E 3 arid is perpendicular to theChord

and on the contrary And hence the Problem, to biffed

an Arch \s folved by drawing a Line AB perpendicular to the Chord in F.

Similar Arches are thofe which contain the fame Num- ber of Degrees of unequal Circles. See Similar Such

are the Arches AB and DE, fig. 87.

Two Radii being drawn from the Centre of two concen- tric Circles ; the two Arches intercepted between 'em, bear the fame Ratio to their refpcclive Peripheries ; and alfo the two Sectors to the Areas of their refpeciive Circles. See Angle.

The Diflance of the Centre of Gravity of an Arch of a Circle, from the Centre of the Circle, is a third Proportio- nal to a third part of the Periphery and the Radius. See Centre of Gravity.

For the Sines, Tangents, &c. of Arches. See Sine, Tangent, f£c.

Arch in Autonomy Diurnal Arch of the Sun, is

part of a Circle parallel to the Equator, defcribed by the Sun in his Courfe betwixt riiing andfetting. See Diurnal, D ay, &c.

His noHurnal Arch is of the fame kind; excepting that it is defcribed betwixt his fetting and riiing. See Night, Rising, i$c.

The Latitude and Elevation of the Pole are meafured by an Arch of the Meridian ; The Longitude, by an Arch of a parallel Circle. See Elevation, Latitude, Lon- gitude, £;c.

Arch ofProgrejfion, or <DireBion, is an Arch of the Zodiac which a Planet feems to pafs over, when its Mo- tion is according to the Order ol the Signs. See Direc- tion.

The Arch of Retrogradation is an Arch of the Zodiac, defcribed while a Planet is retrograde, and moves contrary to the Order oi the Signs. See Retrogradation.

Arch of Station. See Station and S tationary.

Arch between the Centres is an^rc/b,asAI(Tab. Astso- nomy, fig. 35,) pa fling from the Centre ot the Moon's Shadow, A, perpendicular to her Orbit GH.See Eclipse.

If the Aggregate of the Arch between the Centres AF, and the apparent Semi- diameter of the Moon, be equal to the Semi-diamctcr of the Shadow ; the Eclipfe will be total without any duration : if lefs, total with fome dura- tion 5 and if greater, yet lefs than the Sum of the Semi- diameters of the Moon and the Shadow, partial

Arch ofVtfion is the Sun's depth below the Horizon, at which a Star, before hid in his Rays, begins to appear a- gain. See Poetical Risinc.

Arch, in Architecture, is aConcave Building, rais'd with a Mould bent inform of the Arch ofa Curve, and fervin? as the inward Support of any Superstructure. Sec Build- ing.

An Arch, fays Sir Henry Tf'otton, is nothing but a nar- ' row or contracted Vault 3 and a Vault, a dilated Arch. See Vault.

Arches are ufed in large Tntercolumnations of fpaciouS Buildings ; in 'porticos, both within and without Tem- ples 3 in publick Halls, as Ceilings, the Courts of Palaces, Cloitters, Theatres and Amphitheatres. See Portico, Theatre, Ceilinc, $£'c.

They are alfo ufed as ButtrcfTcs and Counter-forts to fupport large Walls laid deep in the Earth, for Foundations of Bridges and Aquajducls, for triumphal Arches, Gates, Windows, &c. See Buttress, h.s.Q-boUtetnts &c.

Arches are either circular, elliptical, or firait

Circular Arches are of three kinds, viz. ift, Semi- circular, which make an exact Semicircle, and have their Centre in the middle of the Chord of the Arch.

idly. Scheme, which are lefs than a Semicircle, and con- fequcntly are flatter Arches 3 containing fome, 90 Degrees, others 7c, and others only 60.

g*dly, Arches of the third and fourth 'Point, as fome of our Workmen call them j tho' the Italians call them di terzo and quarto acuto, becaufe they always meet in an

acute Angle at top Thefe confift of two Arches of a.

Circle ending in an Angle at the Top, and are drawn from the Divifion of a Chord into three or four Parts, at

pleafure- Of this kind are many of t\\Q Arches in old

Gothick Buildings, but on account, both of their Weaknefs and Unfightlinefs, they ought, according to Sir Henry Wotton, to be forever excluded out of all Buildings.

Elliptical Arches confift of a Semi-Ellipfis ■ and were formerly much ufed inftead of Mantle-trees in Chimneys — - Thefe havecommonly'a Keyftone andChaptrcls or Impoftr.

Strait Arches, are" thofe whofe upper and under Edges are firait 3 as in the others they are curved 3 and thofe two Edges alfo parallel, and the Ends and Joints all pointing

towards a Center Thefe are principally ufed over Win*

dews. Doors, &d

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