1456
THEORY AND PRACTICE OF ENGINEERING.
Boor IT.
the joints; to these may be united the moulds of the head, taken from the centre of theelongated face, G c C, of the elevation. The figure in perspective is one of the voussoirsadjoining the key.
Trompe under a Round Tower , in a re-entering Angle . — This is a cone cut by a circularplane, shown on the plan by Gdba C; the voussoirs are divided on the elevation in thequarter circle, B deba C, which presents a section of the cone, through the line B C on theplan.
Fig.2595.
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Fig. 2596. troupe under a round tower.
The projection of the joints of these voussoirs being prolonged on the plan to thatpart of the circle forming the round tewer, and indefinitely on the elevation, to deter-mine the extremity of these joints in the double curve which forms the edge of the centreof the face, draw, as in the preceding, from the points a,b,c,d, lines perpendicular to the axis,till they meet the side A C prolonged. It is evident that those lines are the radii of circleswhich pass through these points : with these radii, describe from the centre A of the ele-vation, sections deba , which cut the corresponding joints in the perpendicular on the plan,indicating the extremities of the joints of the intrados, and those of the curve of thecentre of the face. Knowing the heights and projections, it will be easy to draw theprofile indicated by the development, and the moulds of the intrados and joints, byoperating as in the preceding example.
It must be observed that, whatever be the contour of the trompe on the plan, that is tosay, straight, angular, round, polygonal, or undulated, as in the example at the Chateauof Anet in France , the heights, projections, and elongations of the lines and surfaces arefound in the same manner, by supposing sections parallel to the primitive centre, per-pendicular to the axis of the cone, whatever be the curve of this centre: the operationbecomes longer and more complicated as the contour is more or less regular, or as the coneis more or less oblique. The stone in perspective is one of the voussoirs adjoining thekey.