NATURAL HISTORY BUILDING 63 



intersecting rings of the same size metal, all being surrounded by, 

 and the radial bars secured to, an outer ring .of -|-inch by 8-inch 

 metal. All of these structural bars are enclosed within and support 

 sheet copper muntins, which are so molded as to provide bearing 

 shoulders for the hammered wire plate glass with which the ceiling 

 light is glazed. 



The ledge around the outer side of the base of the inner dome, 

 resulting from the reduction in the thickness of the masonry of the 

 drum, affords a continuous walk, about 3 feet wide, around this part 

 of the rotunda. It is reached from below by iron stairs leading to 

 masonry steps through the drum on the south side, while two door- 

 ways through the drum above the northeast and northwest diagonal 

 sides of the rotunda give access to the pediment roofs. There are 

 also eight openings from the walk through the lower part of the 

 dome, leading to the space back of the main cornice in the interior 

 of the rotunda, for convenience in reaching the electrical installation. 



The outer dome is of heavier construction than the inner one, con- 

 sists partly of a double shell, and the curvature of its two surfaces 

 is not the same throughout. The main shell, which is continuous 

 and furnishes the entire inner surface, springs from the upper cor- 

 belled part of the brickwork of the drum on a line 116 feet 2f inches 

 above the rotunda floor level. It is 73 feet 11 inches in diameter 

 across its base on the inside, and has a tension ring 14 inches by 

 I inch around its bearing. It is five courses of tile in thickness, but 

 its curvature in section is not produced from a dbmmon center. 

 Around the base on the outside an extra shell, 3 courses of tile thick, 

 is added to produce and complete the proper curvature of the outer 

 surface of the dome, which, as in the case of the inner dome, is a 

 spherical segment, the sphere having a radius of 41 feet 8 inches, 

 and its center being 18 feet 10 inches below the bearing of the shell. 

 This radius is increased about 2| inches, by the slate covering. The 

 extra shell is free from the main shell at the base but gradually 

 merges with it. There is a copper-lined gutter extending around the 

 entire upper dome at its base, being partly cut in the top granite 

 course and partly formed along the under edge of the slate covering. 

 It is given proper fall to downspouts which occur at intervals of 

 29 feet and are cut through the shell into the space between the domes. 



In the crown of the outer dome is an eye identical in size with that 

 of the inner dome and fitted with a similar angle compression ring. 

 Around the eye there is a curb of brick, faced on the outer side and 

 above with molded slate courses, which are in turn covered with 

 tinned sheet copper. The height of the curb above the slate covering 

 of the dome is 19 inches, and its greatest thickness is 20 inches. The 

 skylight over the eye is of a different design and construction from 

 the ceiling light of the inner dome, being slightly conical while the 



