MECHANICS AND USEFUL AKTS. 29 



two acres and a half, or exactly 2 52-100 ; in the galleries one acre 

 and 4-4-100 ; total, within an inconsiderable fraction, four acres. 



There are on the ground floor 190 octagonal cast-iron columns, 21 

 feet above the floor, and eight inches diameter, cast hollow, of differ- 

 ent thicknesses, from half an inch to one inch. These columns 

 receive the cast-iron girders. These are 26^ feet long and 3 feet 

 high, and serve to sustain the galleries and the wrought-iron construc- 

 tion of the roof, as well as to brace the whole structure in every direc- 

 tion. The girders, as well as the second story columns, are fastened 

 to the columns in the first story, by connecting pieces of the same 

 octagonal shape as the columns, 3 feet 4 inches high, having proper 

 flanges and lugs to fasten all pieces together by bolts. The number of 

 lower floor girders is 252, besides 12 wrought-iron girders of the same 

 height, and 41 feet span over a part of the nave. The second story 

 contains 148 columns, of the same shape as those below, and 17 feet 

 7 inches high. These receive another tier of girders, numbering 160, 

 for the support of the roofs of the aisles, each nave being covered by 

 16 cast-iron semi-circular arches, each composed of 4 pieces. 



The dome, noble and beautiful in its proportions, is the chief archi- 

 tectural feature of the building. Its diameter is 100 feet, and its height 

 to the springing line is nearly 70 feet, and to the crown of the arch 

 123 feet. It is the largest, as well as almost the only dome hitherto 

 erected in the United States. It is supported by 24 columns, which 

 rise beyond the second story, and to a height of 62 feet above the 

 principal floor. The system of wrought-iron trusses which connect 

 them together at the top, and is supported by them, forms two eccen- 

 tric polygons, each of 16 sides. They receive a cast-iron bed-plate to 

 which the cast-iron shoes for the ribs of the dome are bolted. The 

 latter are 32 in number. They are constructed of two curves of 

 double angle-iron, securely connected together by trellis-work. The 

 requisite steadiness is secured by tie-rods, which brace them both ver- 

 tically and horizontally. At the top, the ribs are bolted to a horizon- 

 tal ring of wrought and cast-iron, which has a diameter of 20 feet in 

 clear, and is surmounted by the lantern. As in the other roofs of the 

 building, the dome is cased with matched deal and tin sheathing. 

 Light is communicated to the interior through the lantern, and also in 

 part from the sides, which are pierced for 32 ornamental windows. 

 These are glazed with stained glass, representing the Anus of the 

 Union and of its several States, and form no inconsiderable part of the 

 interior decoration. 



The external walls of the building are constructed of cast-iron fram- 

 ing and panel-work, into which are inserted the sashes of the windows 

 and the louvres for ventilation. The glass is one-eighth of an inch 

 thick, and was manufactured at the Jackson Glass Works, N. Y., and 

 afterwards enamelled by Cooper & Belcher, of Camptown, N. J. The 

 enamel with which the whole of it is covered is laid upon the glass with 

 a brush, and after drying, is subjected to the intense heat of a kiln, by 

 which the coating is vitrified, and rendered as durable as the glass 

 itself. It produces an effect similar to that of ground glass, being 



