a construction of bricks, stones, or other materials, so arranged that by their mutual pressure and support they bear a superincumbent weight, resting on the two piers or imposts only, and leaving an open space below. They are, however, frequently built in a wall to strengthen it, and then called arches of construction, or merely cut on the surface for ornament: in these cases they are of course not open, and therefore not strictly answering to the definition of an arch, though so called for convenience. Arches are of various forms and denominations; the most common is the round-headed arch, introduced by the Romans, and generally used until the close of the twelfth century: this may be divided into the semi-circular, (fig. 1.) the segmental, (fig. 2.) and the stilted, (fig. 3.)
The triangular arch is found only in a few instances in the UK, as at Barnack, Northamptonshire, and Barton on the Humber, Lincolnshire and is not often found on the Continent: it is always supposed to indicate an early date;
the Convent of Lorsch (above), in Germany, is an example said to be of the 8th c.
ARCHES though not so magnificent as colonnades, are stronger, more solid and less expensive. They are proper for triumphal entrances, gates of cities, of palaces, of gardens and of parks; for arcades or porticos round public squares, markets or large courts, and in general for all apertures that require an extraordinary width. In Bologna, and some other cities of Italy, the streets are on each side bordered with arcades, like those of Covent Garden and the Royal Exchange, which add greatly to their magnificence.
In hot or rainy climates, these arcades are exceedingly convenient to passengers, affording them both shade and shelter; but on the other hand, they are a great nuisance to the inhabitants, as they darken their apartments, hinder a free circulation of air, and serve to harbour idle and noisy vagabonds, who crowd their entrances and disturb their quiet. At Rome, the Courts of the Vatican, those of Monte Cavallo, of the Borghese, and of many other palaces, are likewise surrounded with arcades, where the equipages and domestics attend under cover, some of them being sufficiently spacious to admit two or three coaches abreast.
Such conveniences would be very useful in this metropolis, particularly contiguous to the Court, to the Houses of Parliament, to churches, to all places of public amusement, and even to most town habitations of the nobility and principal gentry, where numerous fine equipages and valuable horses stand half the night, exposed to all weathers. But the scarcity and prodigious value of ground in the fashionable or commercial parts of the town, render them, in general, unattainable.
There are various manners of decorating arches. Sometimes their piers are rusticated, at others they are adorned with pilasters, columns, terms, or Caryatides ; and on some occasions they are made sufficiently broad to admit niches or windows. The circular part of the aperture is either surrounded with rustic arch stones, or with an archivolt, enriched with mouldings, which in the centre is generally interrupted by a key-stone in form of a console, a mask, or some other proper ornament of sculpture, serving, at the same time, as a key to the arch, and as a seemingly necessary support to the architrave of the order. Sometimes the archivolt springs from an impost placed at the top of the pier, and at others from columns with their regular entablature or architrave cornice placed on each side of the arch, and there are some instances of arcades without any piers, the arches being turned from single or coupled columns, sometimes with, sometimes without entablatures ; as in the Temple of Faunus at Rome, and at the Royal Exchange in London, which however is a practice seldom to be imitated, being neither solid nor handsome.
When arches are large the key-stone should never be omitted, but cut into the form of a console, and carried close up under the soffit of the architrave; which by reason of its extraordinary length of bearing, requires a support in the middle. And if the columns that adorn the piers are detached, as in the triumphal arches at Rome, it is necessary to break the entablature ever them, making its projection in the interval no more than if there were no columns at all; for, though the architrave might be made sufficiently solid, yet it would be disagreeable to see so great a length of entablature hanging in the air, without any prop or apparent support.
It is, however, to be remembered that these breaks in entablatures should be very sparingly employed, never indeed but to avoid some considerable inconvenience or deformity; for they are unnatural, render the columns or other supports apparently useless, destroy, in a great measure, the simplicity df the composition, and can seldom be contrived without some mutilations or striking irregularities in the capitals and cornices of the orders, as may be observed in several parts of the inside of St. Paul's in this city, and in many other places.
The imposts of arches should never be omitted; at least if they are, a plat-band ought to supply their place: and when columns are employed without pedestals in arcades, they should always be raised on plinths, which will serve to keep them dry and clean, prevent their bases from being broken, and improve the proportions of the arches, particularly in the Doric order, where the intercolumniations being governed by the triglyphs, are rather too wide for a well proportioned arch.
In all arches it is to be observed that the circular part must not spring immediately from the impost, but take its rise at such a distance above it as may be necessary to have the whole curve seen at the proper point of view. When archivolts are employed without a key or console in their middle, the same distance must be preserved between the top of the archivolt and the architrave of the order as when there is a key, or at least half that distance, for when they are close to each other their junction forms an acute and disagreeable angle.
The void or aperture of arches should never be much more in height, nor much less than double their width ; the breadth of the pier should seldom exceed two-thirds, nor be less than one-third, of the width of the arch, according to the character of the composition; and the angular piers should be broader than the rest by one-half, one-third, or one-fourth. The archivolt and impost must be proportioned to the arch, due care being, however, taken to keep them subservient to the cornice, the architrave, and other principal parts of the order. For this reason the height of the impost should not be more than one-seventh, nor need it ever be less than one-ninth, of the width of the aperture; and the archivolt must not be more than one-eighth, nor less than one tenth thereof. The breadth of the console or mask, which serves as a key to the arch, should, at the bottom, be equal to that of the archivolt, and its sides must be drawn from the centre of the arch. The length thereof ought not to be less than one and a half of its bottom breadth, nor more than double.
The arch of Galerius was built in 298 to 299 AD and dedicated in 303 AD to celebrate the victory of the tetrarch Galerius over the Sassanid Persians and capture of their capital Ctesiphon in 298.
The thickness of the piers depends on the width of the portico, and the weight which the arcade has to carry above, for they must be strong enough to bear the burthen, and to resist the pressure of the porticos vault. But with regard to the beauty of the building, it should not be less than one quarter of the width of the arch, nor more than one-third; and when arches are closed up, to receive doors, windows, or niches, the recesses should be deep enough at least to contain the most prominent parts of what is placed in them; otherwise the architecture will appear flat, and the cornices of the niches or windows projecting before the fronts of the arches, will become too principal and striking in the composition; as may be seen in the second order of the Farnese at Rome.
These dimensions are general, but for a more accurate detail, the annexed designs may be consulted, where the proper measures of every part are expressed in figures.
Vignola in all his orders, excepting the Corinthian, makes the height of the arch double its width. His piers, when the columns have no pedestals, are always three modules, and four modules when they have pedestals; his imposts are all of them one module in height, and the archivolts are either one module, or half a module, as they belong to arches with or without pedestals.
Palladio has given designs only of arches with pedestals. Their height is from one and two-thirds to two and a half of their width, and his piers are all of them nearly three modules and three quarters, excepting in the Composite order, where they are four and four-fifths.
Scamozzi's Tuscan arch is, in height, somewhat less than double its width, which height he increases gradually till, in the Corinthian arch with pedestals, it is nearly twice and one half the width. His piers diminish in proportion to the increase of delicacy in the orders. His Tuscan pier in arches without pedestals being four modules and a half, and his Corinthian only three modules and three quarters. In arches with pedestals, his Tuscan pier is four modules and two-thirds, and his Corinthian only four modules. His imposts and archivolts are likewise varied, and their proportions are relative to the width of the arches and the height of the piers, so that they are considerably larger in arches with pedestals than in those without.
Vignola's arches, being all of the same proportion, do not characterize the difference of the orders. His piers in arches without pedestals are too narrow, and his archivolts too slight In his Doric arch without pedestals, the distance between the arch and architrave of the order is too considerable, as it is indeed in several other of his arches ; and in his Doric with pedestals, the piers are much too broad. Palladio makes too great a difference between the height of his arches. His Tuscan and Doric are too low, his Corinthian and Composite much too high. His piers bear a greater proportion to the void of the arch, in the delicate orders than in the massive. His archivolts are slender, his imposts clumsy and ill profiled.
The apertures of Scamozzi's arches are well proportioned, except in the Corinthian order, where they are, like Palladio's, of an excessive height. His piers bear a proper relation to the arches, as do likewise his imposts and archivolts, excepting in the arches with pedestals, where they are much too predominant in regard to the other parts of his composition, and the members of which they consist are larger than those of the cornice of the order, a fault which Palladio has likewise been guilty of to a very great excess.
At first sight it appears extremely reasonable to augment the size of the imposts and archivolts of arches in proportion to the increase of the aperture, and in cases where no order is employed it ought always to be done; but when the arches are not only adorned with imposts and archivolts, but are likewise surrounded with pedestals, columns and entablatures, it must be very improper to change considerably the proportions of any one of these parte, while all the rest remain unaltered since the consequence must be a considerable disparity between them, so much the more striking, as they are near each other and of similar natures, both circumstances tending to facilitate a comparison ; while a trifling disproportion between the aperture of the arch and its impost or archivolt will seldom be perceived, and never can be very displeasing to the eye.