Chap. XXIV.

MASONIIY.

1 -135

or square with the bed first obtained. When the perpendicular faces are formed applythe mould, and trace out the figure to be given to the voussoirs; the stone is thenreduced by cutting away all extending beyond the outline of the mould, and between thedotted lines. For oblique and circular arches the stones may be traced in a similar manner ;their developcment and profile are shown by the figs, from 2462. to 2474. I3y observingthe letters on each there will be no difficulty in applying the several stones to their respectivepositions.

The Development of Arches , ekher perpendicular , oblique , or circular , on the Plan .—A13 CD is a half cylinder, on which are placed arches extradossed, of an equal thickness,and to which it acts as a newel. Fig. 2475. represents them geometrically ; fig. 2476. theprofile, and in fig. 2477. the whole is shown in perspective at an angle of 45°. The develop-ment of these several arches is expressed in figs. 2478, 2479, 2480, and 2481. ; the voussoirsare shown lying on their extrados, so that the joints appear open at their intrados, wherethey form angles that separate their soffites. The development of the straight arch E F isshown by fig. 2478. ; that composed oftwo parts forming an angle G H I, by fig.

2479. ; the circular L M N by fig. 2480. ;and that which is circular on the planO V Q, but situated obliquely with regardto the axis of the cylinder, by fig. 2481.

To set out these developments, take fromthe profile (fig. 2476.) the width of thefaces of the extrados, which carry on aright line df supposed perpendicular tothe axis in d, 7, 8, 9, 10, 11, 12, and fFor the straight arch E F, it will besufficient to draw a parallel at df at adistance equal to the thickness of thisarch: having then divided each face of the Ig ' ‘ lg ‘

extrados into two equal parts, carry on each side half the width of the internal face, and asit is narrower than that of the extrados, the lines drawn by the points a , 1, 2, 2, 3, 3, 4, 4, 5,5, 6,6, 6, and b, will leave on each side spaces which will represent the joints foreshortened,as shown by fig. 2478.

For the other arches, G HI, LMN, OP Q, the faces of which are oblique or circular,it will be necessary on the horizontal projection, fig. 2475. to draw the lines K R, perpen-

I

Fig. 2478.

dicular to the axis, which may pass at pleasure through one of the projecting extremities,as the point H for the arch GII I, M, for L M X, and R, for O P Q.

li'i it

1 k 1

1 tf. 1

Fig. 2479.

Then prolong the lines from the points of the extrados and intrados until they meet thedirecting line K R : for the arch GHI, for example, make the development for the facesof the extrados, by carrying, as already stated, their width taken on fig. 2475., on theline developed, an> fig. 2479., from a through and n ; having drawn indefinite

perpendiculars through all these points, carry on each the distance of their extremities tothe directing line K R on fig. 2475. ; then carry the distances cd, y7, h 8, iO, A10, 711, 12,