ARC 
machine turns, as the arbor of a crane, 
windmill, &c. 
ARBUTUS, the strawberry-tree, in bo- 
tany, a getms of the Decandria Monogynia 
class of plants, the calyx of which is a very 
small, obtuse, permanent perianthium, divid- 
ed into five segments ; the corolla Consists 
of a single oval petal, divided also into five 
segments ; the fruit is a roundish berry, con- 
taining five cells, and small osseous seeds. 
There are ten species. 
ARC concentric, is that which has the 
same centre, with another arc. 
Arc diurnal, that part of a circle describ- 
ed by a heavenly body, between its rising 
and setting ; as the nocturnal arc is that 
described between its setting and rising: 
both these together are always equal. 
Arcs equal, those which contain the same 
number of degrees, and whose radii arfe 
equal. 
ARCA, in conchology, a genus of Bi- 
valves, the animal of which is supposed to 
be a tethys, the valves are equal, and the 
hinge beset with numerous sharp teeth in- 
serted between each other. 
Arca, in natural history, a genus of 
worms of the order Testacea; animal a 
tethys ; shell bivalve, equivalve ; hinge with 
numerous sharp teeth, alternately inserted 
between each other. There are, according 
to Gmelin, 43 species : but they are sepa- 
rated into four divisions, viz. A. margin very 
entire, beaks recurved ; B. margin entire, 
beaks inflected ; C. margin crenate, beaks 
recurved ; D. margin crenate, beaks inflect- 
ed : of the latter we shall notice A. nucleus ; 
shell obliquely ovate, smoothish, with a 
triangular hiuge ; inhabits European seas, 
and is sometimes found fossile, the size of 
a hazel nut, covered with an olivaceous 
skin, under which it is white, within silvery; 
shell unequally triangular, with very fine 
perpendicular striae, crossed by a few arch- 
ed transverse ones ; depression behind the 
beak, heart-shaped. 
ARCH, or Arc, in geometry, any part of 
the circumference of a circle, or curved 
line, lying from one p'oint to another, by 
which the quantity of the whole circle or 
line, or some other thing sought after, may 
be gathered. 
All angles are measured by arcs. For 
this purpose an arc is described having its 
centre in the point or vertex of the angle : 
and as every circle is supposed to be divid- 
ed into 360°, an arc is estimated according 
to the number of degrees which it contains. 
ARC 
Thus an arc is said to be of 30, 30, or 100 
degrees, &c. 
Arch, in architecture, a concave build- 
ing, with a mold bent in the form of a curve', 
erected to support some structure. Arches 
are either circular, elliptical, or straight, as 
they are improperly called by workmen. 
Circular arches are also of three kinds : 1. 
Semicircular, which have their centre in the 
middle of a line drawn betwixt the feet ot 
the arch. 2. Scheme or skene, which are 
less than a semicircle, Containing some 
90, and some 70 degrees. 3. Arches of thfe 
third and fourth point, consisting of two 
arches of a circle meeting in an angle at the 
top, being drawn from the division of a 
chord into three or more parts at pleasure. 
Elliptical arches consist of a semi-ellipsis, 
and have commonly a key stone and im- 
posts : they are usually described by work- 
men on three centres. 
Straight arches are those used over doors 
and windows, having plain straight edges, 
both upper and under, which are parallel, 
but both the ends and joints point towards 
a center. 
The term arch is peculiarly used for the 
space between two piers of a bridge, in- 
tended for the passage of water, vessels, &c. 
Arch of equilibration, is that which is 
in equilibrium in all its parts, having no 
tendency to break in any one part more 
than in another ; and which is, therefore, 
safer and stronger than any other figure. 
No other arch than this can admit of a 
horizontal line at top : it is of a form both 
graceful and convenient, as it may be made 
higher or lower at pleasure, with the same 
span. All other arches require extrados 
that are curved, more or less, either up- 
wards or downwards ; of these, the ellipti- 
cal arch approaches the nearest to that of 
equilibration for strength and convenience, 
and it is the best form for most bridges, as 
it can be made of any height to the same 
span, its haunches being at the same time 
sufficiently elevated above the water, even 
when it is very flat at top. Elliptical arches 
also appear bolder and lighter, are more 
uniformly strong, and are cheaper than 
most others, as they require less materials 
and labour. Of the other curves, the cy- 
cloidal arch is next in quality to the ellipti- 
cal one, and lastly the circle. 
ARCHANGEL, in botany. See La- 
jviium. 
ARCHES, or Court of Arches, the su- 
preme court belonging to the Archbishop 
