40 UNITED STATES NATIONAL MUSEUM 



lead into the channels. All exposed metal with the exception of the 

 lead gaskets is sheet copper. 



Against the outer end of each of the large skylights there is an 

 opening, 18 feet 10 inches long by 3 feet wide, covered with a wire 

 screen, which serves as an exhaust outlet for the ventilating fans in 

 the attic. 



At the inner end of each of the wings, against the rotunda wall, 

 there is a skylight 38 feet long by 4 feet 6 inches wide, which gives 

 light to the bottom part of the large semicircular windows, and 

 smaller skylights also occur over all the elevator shafts. These sky- 

 lights correspond in detail of construction with the larger ones. 



INTERIOR OF THE SOUTH PAVILION AND THE ROTUNDA 

 GROTmD STORY AND AUDITORIUM 



General 'plan of story. — As before explained, the south pavilion to 

 the height of the adjoining wings is fundamentally square in plan 

 and about 118 feet across, including its enclosing walls which, in the 

 ground story, are 3 feet T^ inches thick on the east and west sides. 

 The structural features which produce the interior octagonal shape 

 are extensions of the walls of the stair towers at the two northern 

 corners, and walls across the southern corners, behind which are sev- 

 eral small rooms and shafts, including the shaft for the south pas- 

 senger elevators. The projection on the south side of the pavilion 

 in the ground story comprises heavy structural walls designed to sup- 

 port the portico and enclosing two larger and two smaller rooms 

 besides a passage to the driveway under the south approach. 



In building the substructure for the large piers of the rotunda, it 

 was found possible to provide for a convenient and commodious 

 auditorium in what would otherwise have been practically waste 

 space, but in so doing it was necessary to extend a part of the floor 

 to a level considerably below that of the ground story. The audi- 

 torium occupies the central part of the area, being surrounded by the 

 substructure piers and intervening screen walls, all of which are of 

 concrete. The piers, which are four in number^ center on the diag- 

 onal lines of the pavilion. They have a maximum width of about 

 40 feet 6 inches and a maximum thickness of about 10 feet, the width 

 across their inner faces being about 36 feet. From the level of the 

 ground floor to within 2 feet of the first floor level they are of hollow 

 construction, having walls 3 feet thick surrounding irregular shaped 

 spaces, in which diaphragms are built at the level of the spring of 

 the auditorium dome, except those in the southern half of the north- 

 em piers which are left open for ventilating purposes. 



