Home >> British Encyclopedia >> Nisthy to Or Brute >> or Arch

# or Arch

## arches, elliptical, line and circle

ARCH, or Ace, in geometry, any part of the circumference of acircle, or curved line, lying from one point 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 di vided into 360°, an arc is estimated ac cording to the number of degrees which it contains. Thus an arc is said to be of 30, 50, or 100 degrees, &c.

Alien, in architecture, a concave build ing, with a mould 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 of the arch. 2. Scheme or skene, which are less than a semicircle, containing sonic 90 and some 70 degrees. 3. Arches of the third and fourth point, consisting of two arches of a circle meet ing 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-ellip sis, and have commonly a key-stone and imposts: they are usually described by workmen 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 centre.

The term arch is peculiarly usedfor the space between two piers of a bridge, in tended for the passage of water, vessels, &c.

A ItC14 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 alai stranger 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 ur_wardsor downwards ; of these, the elliptical 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 be, ing at the same time sufficiently elevated above the water, even when it is very flat at top. Elliptical arches also appear bold er and lighter, are more uniformly strong, and are cheaper than most others, as they require less materials and labour. Of the other curves, the cycloidal arch is next in quality to the elliptical one, and lastly the circle.