VAULT

, in Architecture, an arched roof, so contrived, as that the several stones of which it consists, by their disposition into the form of a curve, mutually sustain each other; as the arches of bridges, &c.

Vaults are to be preferred, on many occasions, to soffits, or flat ceilings, as they give a greater rise and elevation, and are also more firm and durable.

The Ancients, Salmasius observes, had only three kinds of vaults: the first the fornix, made cradlewise; the 2d, the testudo, tortoise-wise, or oven-wise; the 3d, the concha, made shell-wise.

But the Moderns subdivide these three sorts into a great many more, to which they give different names, according to their figures and use: some are circular, others elliptical, &c.

Again, the sweeps of some are larger, and others less portions of a sphere: all above hemispheres are called high, or surmounted Vaults; all that are less than hemispheres, are low, or surbased Vaults, &c.

In some the height is greater than the diameter; in others it is less: there are others again quite flat, only made with haunses; others oven-like, and others growing wider as they lengthen, like a trumpet.

Of Vaults, some are single, others double, cross, diagonal, horizontal, ascending, descending, angular, oblique, pendent, &c, &c. There are also Gothic Vaults, with pendentives, &c.

Master Vaults, are those which cover the principal parts of buildings; in contradistinction from the less, or subordinate Vaults, which only cover some small part; as a passage, a gate, &c.

Double Vault, is such a one as, being built over another, to make the exterior decoration range with the interior, leaves a space between the convexity of the one, and the concavity of the other: as in the dome of St. Paul's at London, and that of St. Peter's at Rome.

Vaults with Compartiments, are such whose sweep, or inner face, is enriched with pannels of sculpture, separated by platbands. These compartiments, which are of different figures, according to the Vaults, and are usually gilt on a white ground, are made with stucco, on brick Vaults; as in the church of St. Peter's at Rome; and with plaster, on timber Vaults.

Theory of Vaults.—In a semicircular Vault, or arch, being a hollow cylinder cut by a plane through its axis, standing on two imposts, and all the stones that compose it, being cut and placed in such a manner, as that their joints, or beds, being prolonged, do all meet in the centre of the vault; it is evident that all the stones must be cut wedge-wise, or wider at top and above, than below; by virtue of which, they sustain each other, and mutually oppose the effort of their weight, which determines them to fall.

The stone in the middle of the Vault, which is perpendicular to the horizon, and is called the key of the Vault, is sustained on each side by the two contiguous stones, as by two inclined planes.

The second stone, which is on the right or left of the key-stone, is sustained by a third; which, by virtue of the figure of the Vault, is necessarily more inclined to the second, than the second is to the first; and consequently the second, in the effort it makes to fall, employs a less part of its weight than the first.

For the same reason, all the stones, reckoning from the keystone, employ still a less and less part of their weight to the last; which, resting on the horizontal plane, employs no part of its weight, or makes no effort to fall, as being entirely supported by the impost.

Now a great point to be aimed at in Vaults, is, that all the several stones make an equal effort to fall: to effect this, it is evident that as each stone, reckoning from the key to the impost, employs a still less and less part of its whole weight; the first only employing, for example, one-half; the 2d, one-third; the 3d, onefourth; &c; there is no other way to make these different parts equal, but by a proportionable augmentation of the whole; that is, the second stone must be heavier than the first, the third heavier than the second, and so on to the last, which should be vastly heavier.

La Hire demonstrates what that proportion is, in which the weights of the stones of a semicircular arch must be increased, to be in equilibrio, or to tend with equal forces to fall; which gives the firmest disposition that a vault can have. Before him, the architects had no certain rule to conduct themselves by; but did all at random. Reckoning the degrees of the quadrant of the circle, from the keystone to the impost; the length or weight of each stone must be so much greater, as it is farther from the key. La Hire's rule is, to augment the weight of each stone above that of the key stone, as much as the tangent of the arch to the stone exceeds the tangent of the arch of half the key. Now the tangent of the last stone becomes infinite, and consequently the weight should be so too; but as infinity has no place in practice, the rule amounts to this, that the last stone be loaded as much as possible, and the others in proportion, that they may the better resist the effort which the Vault makes to separate them; which is called the shoot or drift of the Vault.

M. Parent, and other authors, have since determined the curve, or figure, which the extrados or outside of a Vault, whose intrados or inside is spherical, ought to have, that all the stones may be in equilibrio.

The above rule of La Hire's has since been found not accurate. See Arch, and Bridge. See also my Treatise on the Principles of Bridges, and Emerson's Construction of Arches.

Key of a Vault. See Key, and Voussoir.

Reins or fillings up of a Vault, are the sides which sustain it.

Pendentive of a Vault. See Pendentive.

Impost of a Vault, is the stone upon which is laid the first voussoir, or arch-stone of the Vault. |