] 436
THEORY AND PRACTICE OF ENGINEERING.
Rook II.
and nf, through ad, g 7, h 8, i 9, k 10, 711, m 12, and nf, on the development, fig. 2479.: thenapply the thicknesses d!d", 7 7, 8 8, &c. &c., to the development fig. 2479 from d l throughd”, from 7 through 7, from 8 through 8, &c. &c., andtracing through these points the curvesd '» Wyf) and d", H ;/ , andy /; , we shall have the faces of the extrados.
For those of the intrados, take on the profile fig. 2475. half the difference of the internaland external faces, which will be found by making two parallels to the lines, which passthrough the middle of each voussoir, as 3r and 4 a, with relation to the key.
I 9 1
1 * I
i i t
1 k I
The arch in question being extradossed in equal thickness, the differences 9rand slOare throughout the same, and rs always gives the width of the inferior face; thus to havethe position of the latter, draw 9r through s 10, on the line dn of the development, fig. 2479.through a, a, 1 g, y 1, 1 h, &c. &c., and after having drawn by the points a , and all the points
Fig. 2481.
1, indefinite parallels, carry on each the width of the corresponding lines traced on thecylinder, that is to say, aa, 1 1, 22, 33, &c. &c., of the fig. 2475., through a,a,a', 1,2,1,3,1,4, &c., fig. 2479.
As the lines which terminate each of these faces are in this direction curved, it will benecessary, for greater precision, to take for each a measure in the centre of the face, whichwill give the curve of the opposite sides, by carrying throughout an equal distance dd
To bring together these two faces, and to give them the appearance of reversed voussoirs,draw the lines a ], d, ad", 2,7; 7,2; 3,8; 8,3; 4,9; 9,4; which will represent the joints fore-shortened : the developments of the two other arches, fig. 2480. and 2481. LMN, andO P Q, may be found in a similar manner, the same numbers and letters being used forthe developments of each.
For Skew Arches , inclined in walls circular on the plan. Fig. 2482. is the elevation ;
Fig. 2483.
Fig. 2488.