52 UNITED STATES NATIONAL MUSEUM 



upon the ends of a wall corresponding in relative position to the 

 other three pediment walls. The beams from the truss to the drum 

 and pendentive masonry, a distance of 14 feet 3 inches, are 10 inches 

 deep, and those from the truss to the outer wall, a distance of 16 feet 

 2 inches, are 12 inches deep. Saddles with valley beams 18 inches 

 deep occur on the diagonal lines between the roof slopes. The frame- 

 work is overlaid with an 8-inch slab of reenforced concrete, and this 

 in turn is covered with slate of the same quality and color as has been 

 used on the wings and ranges. 



EOTUNDA DOMES 



The covering of the rotunda consists of an inner and an outer dome 

 of Guastavino construction, the former constituting the ceiling, the 

 latter the roof of this part of the building. Both of these domes 

 rest, independently of each other, on the brick masonry of the drum 

 which is entirely concealed from view. The drum begins on a line 

 with the top of the large limestone arches, 85 feet 11 inches above the 

 floor level of the rotunda, where it has an inside diameter of 71 feet 

 10 inches and is 6 feet 10 inches thick. Eetaining these dimensions 

 for a height of 8 feet 3 inches, it then becomes abruptly recessed on 

 the inside and reduced in thickness to 3 feet 9 inches, which is never 

 exceeded above except where certain courses of stone project beyond 

 the general outside surface of the drum. The total height of the 

 brickwork of the drum is 30 feet 3f inches, the outer stone facing 

 passing above its summit to the extent of only one narrow course. 



The inner or ceiling dome rests at its edge upon a corbel from the 

 inner angle at the top of the basal or thicker part of the drum, back 

 of the main cornice, a tension ring being provided around the bear- 

 ing of the shell. Two rows of buttresses 6 inches wide, spaced 10 

 feet on centers, also brace the dome from the drum. In shape, the 

 dome is a spherical segment, the sphere from which it is derived 

 having a radius of 35 feet 11 inches, and the center being 5 feet 

 below the spring line of the dome. It falls somewhat short, there- 

 fore, of being a complete hemisphere, and its diameter at the base 

 is 71 feet 6 inches. 



In construction the inner dome consists of three courses of Guas- 

 tavino tile in thickness, the inner course, laid in herringbone pattern, 

 presenting a buff-colored unglazed surface as a ceiling. At the center 

 there is a circular opening or eye for admitting light, 18 feet 10 

 inches in diameter, the edge of the shell bearing against a steel com- 

 ])ression ring and the opening being finished on the under side with 

 a terra cotta molded ring. The ceiling light covering the eye is 

 concentric with the dome, but derived from a sphere of slightly 

 greater radius. It is divided into a number of component lights 

 by twenty-four |-inch by 3-inch radial bars and seven concentric 



