PLATE IV. Fly are 1.—Describe every rib with a trammel, by taking the extent of each base, from the plan whereon the ribs stand, to its centre, and the he ght of each rib to the height of the top of the niche; it will give the true sweep of each nb.

To euoye the ribs of the niche.—Therc will be no occasion for making any moulds fir these ribs, but make the ribs themselves; then there will be two ribs or each kind : take the small distances I from the plan at n, and put it to the bottom of the ribs D Mid E, to 2, and e to 1 ; then the ranging may be drawn Off by the other corresponding with the ; as for• example, at the rib E, by moving the centre of the trammel towards e, upon the line e c, from the centre, c, equal to the distance 1 e, the tram mcd-rod remaining the same as when the inside of the curve was struck.

Given one of the common ribs of the bracketing of a rove, to land the angle-bracket for a rectangular room.

Figure 2.—Let 11 be the common bracket, b c its base ; draw b a perpendicular to he, awl equal to it draw the hypo thenuse a c, which %yin lie the place of the mitre; take any number of ordinates in n, perpendicular to 6 c, its base, and continue them to meet the mitre line, a c, that is, the base of the bracket, at ; draw the ordinates oft at right angles to its base; then the bracket at 1, being pricked born 11, as be seen by the figures, will be the form of the angle-rib required.

Xote,—The angle-rib must be ranged either externally or internally, according to the angle of the room.

NienE, .ilogo/ur, one formed in the corner of a building. NtenE, Cul de Four of u. ,See Cee DE FOUR.