1472
THEORY AND PRACTICE OF ENGINEERING.
Rook II.
mence by raising from all the points of the lines pg and gp on the plan perpendiculars,and one from the point b, on which take a point a to represent the lower re-enteringangle of this voussoir; having set off above r, and below k, the height of the slope of
Fig. 2668.
Fig. 2670.
Fig. 2669.
the ascending and descending parts, draw through the two points r and k horizontal lines,which determine the points A and A on the elevation, by their intersection with perpendicu-lars raised from the corresponding points on the plan : then draw the inclined line A a A,to represent the apparent arris of the lower joints of the voussoir.
To obtain the curvature of the descending branch of this voussoir from the point a, takethe heights A,2,S,4,p,n, indicated on the perpendicular joint gh of the centre ; set them offon the perpendiculars raised through the corresponding points of M, beginning from thehorizontal line of the base, passing through the point A.
For the ascending branch take the heights on the upright joints g, A of the centre, whichwill be found similar to those indicated by the letters A, 8,9, < 7 , n of the section, which wereset off in the same manner on the perpendiculars raised on figure M, commencing from theupper horizontal line passing through rh.
Find the divisions on the perpendicular lines gh,gh, which mark the joints, by drawingarrises, 7 a, 12, from the profile of the second voussoir, to those corresponding to theupper centre, 2 a, 6 6 , of the lines which cut the vertical projection of these joints. Thisoperation is founded on the idea that the arrises of the rows of voussoirs on the plan formright lines from the centre of the diagonal B F to that of the diagonal GC. ^ e mustobserve that the elongated centres of these diagonals are represented on the elevation bysemicircles, because the plane of projection on which they are expressed is parallel to theplanes of the semicircles from which they originate, and which form the primitive centre oframpant vaults.