1346
THEORY AND PRACTICE OF ENGINEERING.
13ook II.
trusses, is filled in with two half trusses of the same kind, and four smaller ones, whichare sustained by great braces in their proper position.
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Fig. 2168.
Dome of the Assumption, at Paris , designed by Errard, is an example where the interiormay be rendered available to effect, and to the purposes of decoration; its principles are laiddown by M. Stierme, in KraflYs work on carpentry : two trusses cross each other at rightangles in the middle, and these, with twelve others arranged intermediately around thedome, all converge to the centre, and carry the lantern. The principle adopted in thesetting out was to form first a square, the side of which was determined by the heightto the under side of the lantern : where the diagonals of the square so set out crosseach other a point was established, determining the radius of the inner dome, which isperfectly hemispherical : this point is found by elongating the line of face of the inner wall,until it intersects the diagonal, establishing the height at which the horizontal tie isto be placed. The inclined posts, which carry the chief weight, are tangents to the curveof the inner dome, and are each framed into two struts, that support the outer rib of thedome, which carries the covering : five horizontal and concentric ties, at regular andparallel distances, maintain these four principal ribs in their true position, and, beingframed into them, bind them together.
The ('hnrch of St. Mark , at Vtnice, exhibits a dome of very early construction, its datebeing assigned to the year 1085 ; after its erection some additional timbers were intro-duced by Giacomo Sansovino, in 1530, which act as inclined shores to bear up thecupola: the outer diameter is about 51 feet, and its height 60: it forms a coveringto a brick vault beneath it. The lower portion is cylindrical, and i> framed throughoutits circumference with upright posts, set at a distance of about 2 feet from centre to centre;they are united at the top and bottom by circular plates. The hemispherical portion otthe dome is composed of stout double planks, so put together that each forms a sort of rib;these are united by four horizontal ties placed at regular distances : the exterior iscovered with planking bent round to receive the lead.