A line extending from any point in the springing line on one side of the arch, to the springing line on the opposite side of the arch, is called the chord or span of the arch.
If a vertical plane be supposed to be contained by the span and the intrados of the arch, it is called the section of the hol low of the arch.
The vetrieal line drawn on the section from the middle of the spanning line to the intrados, is called the height of the arch, as also the middle line of the arch ; and the part of the arch at the upper extremity of this line is called the crown of the arch.
Each of the curved parts on the top of the section, between the crown and either extremity of the spanning line, is called the haunches or ji«nks of the arch.
The section of almost every given arch used in building has the following properties : the upper part is one continued curve, concave towards the span, or two curves forming an interior angle at the crown, both concave towards the span ning line. Every two vertical lines on the section equi distant front each extremity, and parallel to the middle line, arc equal.
The foregoing definitions and propositions not only apply to arches with level bases, but also to arches which stand upon inclined bases.
When the base of the section or spanning line is parallel to the horizon, the section will consist of two equal and similar parts, so that if one were conceived to be folded upon the other, the boundaries of both would coincide.
Arches, whose intrados is the surface of a geometrical solid that would fill the void, are variously named, according to the figure of the section of that solid perpendicular to the axis. as circular, elliptical, cycloidal, catenurian, paraboli cal. &c.
Arches of the circular kind have two distinctions, viz., the semicircular, and those of segments less than a semicircle; the latter are called scheme or skene arches.
There are also pointed, composite, lancet, or Gothic arches, which are formed in the face of the wall, or in sections parallel thereto, with the intrados of the arch.
When the extremities of an arch rise from supports at unequal heights. it is called a my/timpani arch.
When vertical lines are drawn upwards, through each extremity of the spanning line, so as to cut off equal and similar parts of the intrados, the arch is called a ho•se-shoe arch, or nioresque arch. Hence, in this kind of arch, the spanning line is less than any other line or chord drawn parallel to the span, but under the top of each of the vertical lines.
When the upper line or side of an arch is parallel to the under line or side, it is called an extradossed arch.
A simple vault is an interior concavity extended over two parallel opposite walls, or over all the diametrically opposite parts of one circular wall. Au arch or vault is frequently understood as synonymous; but the distinction which we shall here observe is, that an arch, though it may be extended over any space, has a very narrow intrados, not exceeding four or live feet ; whereas a vault may be extended to any limit more than four or five feet. Thus we frequently say an arch in a wall, but we never say a vault in a wall ; though nothing is more common than to say a vaulted apartment, a vaulted room, a vaulted &c. So that a vault, as Sir Henry Wotton has observed, is an extended arch ; we shall therefore apply arch to the head of the aperture in a wall which shows curvilinear intersections with the faces of the wall, and the word vault to arched apartments. We frequently, however, call the stone-work suspended over an apartment an arch as well as a vault ; so that every vault is an arch, but every arch is not a vault.
The intrados of a simple vault is generally formed of the portion of a cylinder, cylindroid, sphere, or spheroid, never greater than the half of the solid ; and the springing lines which terminate the walls, or where the vault begins to rise, are generally straight lines parallel to the axis of the cylinder, or cylindroid, or the circumference of a circle or ell ipsis.
A circular wall is generally terminated with a spherical vault, which is either hemispherical, or a portion of a sphere less than a hemisphere.