VOUSSOIRS

, vault-stones, are the stones which immediately form the arch of a bridge, &c, being cut somewhat in the manner of a truncated pyramid, their under sides constituting the intrados, to which their | joints or ends should be every where in a perpendicular direction.

The length of the middle Voussoir, or key-stone, and which is the least of all, should be about 1/15th or 1/16th of the span of the arch; from hence these stones should be made larger and larger, all the way down to the impost; that they may the better sustain the great weight which rests upon them, without being crushed or broken, and that they may also bind the firmer together.

To find the just length of the Voussoirs, or the figure of the extrados, when that of the intrados is given; see my Principles of Bridges, or Emerson's Construction of Arches, in his volume of Miscellanies.