Chap. XXIV.

MASONRY.

1467

On the section is shown the position of the various arcs numbered on the plan, whichtend to the centre of the sphere, of which the triangle D A G is a portion. The figure inperspective is a voussoir of the fourth bed.

Another method by Voussoirs in the manner already described for the Construction of a Trompe.

.—The surface of these pendentives are portions of arcs of circles, but instead of being com-prised in planes tending to the axis of the dome, they are contained in those which unitein the interior of each pillar, at a vertical line whose projection is the point K : in orderthat this should produce a good effect, the joints are not divided on the entire arc C D ofthe plan, but on the part between C and M, to avoid the too great thinness of the edgesof the row of voussoirs joining A D; the part M D may be a fourth or fifth of the arc C D ;in the present instance it is rather less than the fourth.

Fig. 2654.

Fig. 2652.

PENDENTIVES.

To draw the projection of these joints commence by dividing C M into nine equal parts,one of which is given to half the key, and two to each of the other voussoirs: then havingprolonged AD, till it cuts CB prolonged in K, draw through these points to 1,2,3,4,right lines which cut AB in the points 5,6,7,8. The lines 15,2 6, 3 7,48, express theprojection of the chords of the arcs which form the upright joints of the voussoirs: by settingoff these on the elevation, draw the chords in a similar manner, but neither thoseon the plan nor on the elevation give their true length, by reason of their double obliquity;to have this, after having drawn the horizontal line CD, raise a perpendicular to it, O B,equal to B C, and set off on C D the distances O 1, 0 2, O S, 04, O D, equal to B C, 51,62,73,84, 8D on the plan; then draw the lines B C, Bl, B2, &c. &c., which will be thechords of the arcs required: these centres may be found by raising on the middle of eachperpendiculars which will cut B N in 5,6,7, and 8, which will be the centres of the arcscorresponding to each of these chords. Then having prolonged the lines 17, 18,19, &c. &c.from the horizontal joints of each voussoir in the section, in such a manner that they cut thearcs just traced, these intersections will serve to find the projection of the plan of the hori-