ARCH, an arrangement of bricks, stones, or other materials over an open space, by which they are made not only to support each other by mutual pressure, but to sustain a superincumbent weight. We have the excellent authority of Sir G. Wilkinson for stating, that the A. was known to, and used by, the ancient Egyptians; and that the Assyrians were acquainted with its principles is placed beyond doubt by the arched gateways so frequently represented in their bass-reliefs. The A. is generally supposed to have been unknown to the Greeks—a supposition which becomes very improbable if we hold it to be proved that it was used by nations with whose works they must have been familiar. But that the Greeks did not employ it generally in their architectural struc tures is certain; and as it is not less certain that the Romans did, it is to the latter people that the nations of modern Europe are indebted for their acquaintance with its great utility. The introduction of the A. by the Romans gradually effected a complete revo lution in the architectural forms which they borrowed from the Greeks. The predomi nance of horizontal lines gave way by degrees, till, as the Romanesque passed into the Gothic style, it was superseded by the segments of a circle, placed generally more or less in a perpendicular direction. In its earliest application by the Romans, the A. did not spring from the entablature of the columns, but was generally placed behind them, and rested upon separate imposts. Subsequently, this arrangement was departed from, and the A. assumed the position which it has since retained above the columns; sometimes having an entablature interposed, and sometimes rising directly from the capital of the column or pier, as in the Romanesque. Before mentioning very briefly the different forms of the A., it seems natural to refer to a very simple structure, frequently met with in those early edifices in our own country which we are in the habit of designating as Saxon. It consists of two stones, their lower ends resting on rude piers, their tops leaning against each other, and thus forming two sides of a triangle, which is capable of supporting a moderate superincumbent weight. The mechanical principles on which the A. depends, though here very imperfectly employed, seem sufficiently called into play to suggest their more extensive application; and it is not impossible that out of this rude construction the A., in its later and more elaborate forms, might have developed itself amongst ourselves without hints from foreign sources.
Of the A. itself, the following variations of form may be enumerated: The semi circle, the segment, the ellipse, which were the only forms employed by the ancients, and which alone were known in mediteval architecture up to the time at which the pointed A. was introduced. Of these, the stilted A. and the horseshoe A. are modifica
tions, in both of which the center or point from which the A. is described is above the line of the impost, but in the former of which the moldings are continued downwards vertically; whilst in the latter they are slightly inclined inwards, or the curve is pro longed till it meets the impost. The horseshoe A. belongs peculiarly to Arabian archi tecture (q.v.),.not only from its having originated simultaneously with the faith of the prophet, but from its continuing to be used exclusively by his followers. Next, in point of time, though far surpassing all the others in beauty and variety, is the pointed A., the origin of which is still a subject of antiquarian controversy.
The greater or less acuteness of the pointed A. depends on the position of the two center points from which its curved sides b I are described.
Of the foil arches or arches in which the forms of a leaf are imitated may be mentioned the trefoil, the cinquefoil, and the a polyfoil, tile latter being met with in Arabian and Romanesque buildings. At a later period of Gothic architecture, with the decorated style, the ogee A. was introduced, and the Tudor or four-cornered A. appeared about the commencement of the per pendicular style. When first introduced, the proportions of this A. were bold and effective; but it was gradually depressed till the principle of the A. was lost, and its very form was again merged first in two and then in one flat stone or lintel over an opening. With the last form of the Tudor A. we thus reach almost the point of departure in the construction of the A., and complete our enumeration of its forms, The sides of an A. are termed haunches or franks; and its highest part is called the erolva. The wedge-shaped stones, bricks, or other materials of which an A. is ma i; K. I.-40 structed are called voussoirs (a, a, a); the uppermost one of all (b) is called the keystone; the lowest, which is placed immediately over the impost, the springer, or springing-stone; the under or lower side of the voussoirs, the Intrados; the upper side, the extrados or back. For the investigation of the mechanical principle of the A., and of the conditions 1 of stability, see Moseley's Mechanical Principks of Engineering and Architecture. Sea . also BRIDGE, IMPOST, PIER, BUTTRESS.