arc-cosine 
arc-COSine (ark-ko'sm), . In math., an angle 
regarded as a function of its cosine. 
arc-cotangent (ark-ko-tau'jent), n. In math., 
an angle regarded as a function of its cotangent. 
Arcella (ar-sel'ji), . [NL., dim. of L. area, a 
box : see area, J : A' 2 .] A genus of amoeboid pro- 
tozoan organisms having a kind of carapace or 
shell, the type of a family ArcelUilte. 
Arcellidae (ar-sel'i-de), ii.pl. [NL., < Arcella + 
-idtc.] A family of Protozoa, of the order Amae- 
hoidca, containing the genera Arcella, J)i(ttu<//<i, 
etc., the members of which are inclosed in a 
kind of test. 
arch 1 (iirch), 11. [< ME. arch, i/rcltc, < OF. arche 
(> ML. arcliia), mod. F. arche, an arch, fern, 
form (prob. by confusion with OF. and F. 
arche, ark, < L. area : see arch 2 ) of OF. and 
F. arc, < L. areas: see arc*.] If. In guom., 
any part of the circumference of a circle or 
other curve; an arc. See arc 1 , 1. 2. Inarch., 
a structure built of separate and inelastic 
blocks, assembled on a curved line in such a 
way as to retain their position when the struc- 
ture is supported extraneously only at its two 
extremities. The separate blocks which compose the 
arch are called voussoirs or arch-stones. The extreme or 
lowest voussoirs are termed springers, and the uppermost 
or central one, when a single stone occupies this position, 
is called the keystone. The under or concave face of the 
292 
pant. Foil arches are arches whose intrados outlines form 
a series of subordinate arcs called foils, the points of which 
Cycloidal Arch. 
Elliptical Arch. 
Horseshoe Arch. 
< i^cc An It. 
Equilateral Arch. 
Exrradosed Arch. 
a, abutments; -v, voussoirs; s, springers; i, imposts; /, intrados; 
/, piers ; k, keystone ; Ex, extrados. 
assembled voussoirs is called the intrados, and the upper 
or convex face the extrados, of the arch. When the curves 
of the intrados and extrados are concentric or parallel, 
the arch is said to be extradosed. The supports which 
afford resting and resisting points to the arch are piers or 
pillars, which receive the vertical pressure of the arch, 
and abutments, which resist its lateral thrust, and which 
are properly portions of the wall or other structure above 
the springing and abreast of the shoulder of the arch. 
The upper part of the pier upon which the arch rests 
(technically, the point from which it springs) is the im- 
post. The span of an arch is the distauce between its 
opposite imposts. The rise of an arch is the height of 
the highest point of its intrados above the line of the 
imposts ; this point is sometimes called the under side of 
the crown, the highest point of the extrados being the 
crown. The thrust of an arch is the pressure which it 
exerts outward. This pressure is practically collected, so 
far as it is manifested as an active force, at a point which 
cannot be exactly determined 
theoretically, but is at about 
one third of the height of the 
rise of the arch. The thrust 
must be counteracted by abut- 
ments or buttresses. Arches 
are designated in two ways : 
skew Arch. First, in a general manner, 
according to their properties, 
their uses, their position in a building, or their exclusive 
employment in a particular style of architecture. Thus, 
are termed cusps. A numeral is usually employed to desig- 
nate the number of foils, as a trefoil m-rli, :i ciinju^foii 
arch, etc. 
3. Any place covered with an arch or a vault 
like an arch : as, to pass through the arch of a 
bridge. 4. Any curvature in the form of an 
arch: as, the arch of the aorta; the arch of an 
eyebrow, of the foot, of the heavens, etc. 
Whereon a sapphire throne, inlaid with pure 
Amber, and colours of the showery arch. 
Milton, P. L., vi. 759. 
5. In mining, a portion of a lode left standing, 
either as being too poor for profitable working 
or because it is needed to support the adjacent 
rock. 6. The roofing of the fire-chamber of 
a furnace, as a reverberatory or a glass-fur- 
nace ; hence, sometimes, the fire-chamber itself. 
Alveolar arch, aortic arch. Sec the adjectives. 
Arch of discharge, an extradosed arch built in the ma- 
sonry of a wall, over a doorway or any other open or weak 
place, to transfer pressure from 
above to points of assured sta- 
bility on either side. An arch 
of discharge is generally distin- 
guishable to the eye from the 
wall in which it is built mere- 
ly by the position of its stones, 
or at most by a slight projec- Arch of Discharge, 
tion beyond the wall-surface. (From Viollet-le-Duc's "Diet. 
Arch Of the fauces. See del' Architecture.") 
fauces. Axillary arches. 
See axilla n/. Back of an arch. See 6<ic*i. Backing 
of an arch. See backimj. Basket-handle arch, an el- 
liptical arch, or a three-centered low-crowned arch. Blind 
arch, an arch of which the opening is walled up, often used 
as an arch of discharge. See cut under arcature. Bran- 
chial arch. See branchial. Clustered arch, a number 
of arched ribs springing from one impost, a form usual in 
medieval pointed vaulting. See cut under (clustered) col- 
umn. Court of Arches. See court. Crural or In- 
guinal arch. See crural. Flat arch, an arch of which 
i the intrados is straight, the vonssoirs being 
wedge-shaped and assembled in a horizon- 
tal line : used especially in brickwork, 
where the charge to support is not great. 
Hemal arch, hydrostatic arch. Seethe 
adjectives. Laminated arch, a beam ill 
Flat Arch. the form of an arch, constructed of several 
thicknesses of planking bent to shape and 
bolted together : a form of arched beam. Mandibular 
arch, mural arch, neural arch, etc. See the adjec- 
tives. Oblique arch. Same as skew arch (which see, 
below). Pectoral arch. Same as pectoral girdle (which 
see, under girdle). Pelvic arch. Same as pelvic girdle 
(which see, under girdle). Preoral arches, postoral 
arches. See the adjectives. Recessed arch, one arch 
within another. Such arches are sometimes called double, 
triple, etc., arches, and sometimes compound arches. Re- 
versed arch, an inverted arch. Ribbed arch, an arch 
composed of parallel ribs springing from piers or imposts. 
Rough arch, anarch formed of bricks or stones roughly 
dressed to the wedge form. Round arch, a semicircular 
arch. Skeletal arches. See visceral arches, under vis- 
ceral. Skew arch, an arch of which the axis is not per- 
pendicular to its abutments. Stilted arch, an arch of 
which the true impost is higher than the apparent im- 
Segmental Arch. Semicircular Arch. 
there are arches of equilibration, equipollent arches, arches 
of discharge, skew and reversed arches, Roman, Pointed, 
and Saracenic arches. Second, they are named specifically, 
according to the curve the intrados assumes, when that 
curve is the section of any of the geometrical solids, as 
segmental, semicircular, cycloidal, elliptical, parabolical, 
Stilted Arches. Modem Romanesque. 
post, or of which the piers are in fact continued above 
the apparent impost, so that a portion of the intrados on 
either side is vertical. Surmounted arch, a stilted 
semicircular arch ; a semicircular arch of which the rise 
is greater than the radius. Triumphal arch, a monu- 
mental arch in honor of an individual, or in commemo- 
ration of an event. Such arches were first erected under 
hyperbolical, or catenarian arches; or from the resem- 
blance of the whole contour of the curve to some familiar 
object, as lancet arch and horseshoe arch ; or from the 
method used in describing the curve, as equilateral, three- 
centered, four-centered, ogee, etc. When an arch has one 
of its imposts higher than the other, it is said to be ram- 
Triumphal An 
archaean 
the Roman emperors, ami were originally temporary 
structures, festooned and otherwise decorated, standing 
at the entrance of a city, or in a street, that a victorious 
general and his army might pass under them in triumph. 
At a later period the triumphal arch became a richly 
sculptured, massive, and permanent structure, having an 
archway passing through it, and often a smaller arch on 
either side. The name is at the present day often given 
to an arch, generally of wood decorated with flowers, 
evergreens, banners, etc., erected on the occasion of some 
public celebration or rejoicing. The great arcli in a church 
which gives access to the choir the chancel arch is 
sometimes so called. In early Christian churches, a rep- 
resentation of the Glory or Triumph of Christ sometimes 
occupied a wall-space above this arch. 
Statues, and trophies, and triumphal area, 
Gardens, and groves, presented to his eyes. 
Milton, P. K., iv. 87. 
Twyer arch, an arched opening in a smelting-furnace to 
admit the blast-pipes. Tymp arch, the arch above the 
tymp in a blast-furnace. See tymp. Vascular arches. 
See visceral arches, under visceral. Visceral arches. 
See visceral. 
arch 1 (arch), r. [< arch 1 , .] I. trans. 1. To 
cover with a vault, or span with an arch. 
The proud river . . . is arched over with . . . a curlSus 
pile of stones. Howell. 
No bridge arched thy waters save that where the trees 
stretched their long arms above the'e and kissed in the 
breeze. W'lnttier, Bridal of Pemiacook. 
2. To throw into the shape of an arch or vault ; 
curve : as, the horse arches his neck. 
Fine devices of arching water without spilling. 
Bacon, Gardens. 
Beneath our keel the great sky arched 
Its liquid light and azure. 
11. P. Spoford, Poems, p. 11. 
II. intrans. To form an arch or arches: as, 
the sky arches overhead. 
The nations of the field and wood . . . 
Build on the wave, or arch beneath the sand. 
Pope, Essay on Man, iii. 102. 
arch 2 ! (arch), n. [< ME. arclie, in Scriptural 
senses, assibilated form of arc, ark, < AS. arc, 
earc, ere (see ark 2 ), merged with the identical 
OF. arche, airche, < L. area, a box, chest : see arc 2 , 
rf' 2 .] 1. A box or chest; in plural, archives. 
The civile law . . . was laid up in their arches. 
Holland, tr. of Livy, IX. xlvi. 349. (N. E. D.) 
2. The ark of Noah. [The common form in 
Middle English.] 3. The ark of the covenant. 
arch 3 (arch), a. and n. [A separate use of the 
prefix arch-, chief, which in many compounds 
has acquired, from the second member of the 
compound, or from the intention of the user, 
a more or less derogatory implication.] I. a. 
1. Chief; principal; preeminent. See arch-. 
The tyrannous and bloody act is done ; 
The most arch deed of piteous massacre 
That ever yet this land was guilty of. 
SAaJt.,Kich. III., Iv. 8. 
Died that arch rebell Oliver Cromwell, call'd Protector. 
Evelyn, Diary, Sept. 8, 1658. 
2. Cunning; sly; shrewd; waggish; mischiev- 
ous for sport ; roguish : now commonly used of 
facial expression: as, "so arch a leer," Toiler, 
No. 193. 
He had the reputation of an arch lad at school. Suift. 
So innocent-arcA, so cunning-simple 
From beneath her gather'd wimple 
Glancing with black-beaded eyes. 
Tennyson, Lilian. 
Tile archest chin 
Mockery ever ambush'd In ! 
M. Arnold, Switzerland. 
II. t n. A chief; a leader. [Rare.] 
The noble duke my master, 
My worthy arch and patron, comes to-night. 
Shak., Lear, ii. 1. 
arch-. [X ME. arch-, arche-, etc., < AS. arce-, 
also erce- and arce-, = D. aarts- = OHG. erzi-, 
MHGr. G. erz- = 8w. arke-, erkc- = Dan. arrke-, 
erke- = Bohem. arci-, archi- = Pol. arc;/-, archi- 
= Russ. arkhi- (ME. arche- also partly < OF. 
arce-, arche-, mod. F. arch-, arc/ii- = Pr. Sp. Pg. 
arce- (Sp. sometimes arzo-) = It. arce-, arci-), < 
L. archi- (= Goth, arfc-in arkaggiliis, archangel), 
< Gr. ap%i-, apx-, combining form of ap^of, chief, 
< ap%civ, be first, begin, lead, rule, = Skt. \/ arh, 
be worthy.] Chief; principal: a prefix much 
used in composition with words both of native 
and of foreign origin. See a'c7|3. 
archabbot (iirch' / ab'ot), . [< arch- + abbot.'} 
A chief abbot : applied as a specific title to the 
head of certain monasteries. 
archaealt (ar-ke'al), a. [< arcluxns + -?.] 1. 
Pertaining to the'archreus, or supposed internal 
cause of all vital phenomena. 2. Caused by 
thearchseus: as, arch(eal diseases. Seearclia'im. 
archsean (iir-ke'an), a. [< Gr. apxaloc,, ancient : 
see archteo-.] Of or relating to the oldest 
period of geological time .- a name proposed 
by J. D. Dana, and now generally adopted, for 
a series of crystalline schists and massive 
rocks lying underneath the most ancient fos- 
