1618
THEORY AND PRACTICE OF ENGINEERING.
Book II.
more recent conical and hemispherical domes are placed: the intersection of the twogreat triangles fixes the diameter to be given to the internal void, around which the sideaisle, its walls and pillars should be formed. The circle which has its diameter com-prised between the apex of the two equilaterals determines the clear width between the
Fig. 3027.
outer walls. That the architects of those days delighted in the forms produced by theseveral intersections of the circle in combination with the equilateral triangle, we areassured by viewing the several designs they have left us in mosaic upon the walls of theDuomo, and at the cathedrals of Florence , Sienna, and elsewhere.
Roslyn Chapel, Scotland , commenced about the year 1446, has its buttresses wellsuited to give aid to the walls, and to enable them to resist the thrust of its nearly semi-circular vault, which they receive below the springing. The extreme width from faceto face of the buttresses is 48 feet 4 inches; the span of the nave is 15 feet 8 inches,being 5 inches less than the proportion of a third; the two side aisles together