1474

THEORY AND PRACTICE OF ENGINEERING.

Book IT.

the ellipses which pass through the upper extremities of the joints of the trompes, anclwhich determine their elevation; they are drawn by sections described from their foci.The semi-greater axis of each quarter ellipsis is found by drawing on the plan parallelsto IP, through the extremities I, 1, 2, 3, 4, and K, of the joints: the semi-lesser axesare obtained by setting off on the section the distances Bi, 155, BG, 13 7, 13 8, 139, 13 K ;from each of these points perpendiculars are raised to the intersections of the line B K,which represents the profile of the cone in its centre, and the height Kk is made equalto the breadth I K of the ramps.

Another Method* with an Arc de Cloitre under the Quarter Space, whose Springing is level _

The centre of the voussoirs is formed of portions of an ellipsis : to obtain them, describe onN A prolonged an arc of a circle A G, whose semi-chord G N is twice the width of theramp III; this semi-chord is divided into seven equal parts, through which ordinatesare drawn parallel to N A: having then drawn the lines AN,NG, which form a rightangle, both equal to N A, fig. 2G77., divide N G into the same number of parts, as N G;and after having drawn parallels to AN, set ofF the length of the ordinates on fig. 2677.,as on fig. 2678., by means of which describe a quarter ellipsis corresponding to thearc G A. The perspective figure is the voussoir I of the section.

8 9 o~

17 i

Fig. 2679. plan.