218 HISTORY OF ASTRONOMY. 



tions of the building to a height of 90 feet, terminating 

 about one foot above the ceiling of the fourth story of the 

 tower. They are tied together at each floor by wooden 

 beams, and at the top by four cast-iron girders a pair, 

 four and a half feet apart, being placed near each end. . 



The observing room is 24 feet square, and is covered 

 by a flat roof with the exception of the part occupied by 

 the equatorial. 



The eastern pair of iron girders are framed together 

 midway between the piers, and upon this frame-work is, 

 constructed of bricks and cement, a prism, four feet 

 square at the base, reaching nearly to the floor, and from 

 this point a cylinder, three feet in diameter, rises three 

 feet above the floor. This cylinder is incased in a drum 

 of boiler iron. The marble stand of the telescope is im- 

 bedded to a depth of 18 inches in the cylinder, and rises 

 to a height of six feet and three quarters above it. The 

 equatorial is covered by a hemispherical dome of 12 feet 

 in diameter, constructed upon the plan of that of Mr. 

 Campbell's observatory in New York, with revolving 

 table and steps attached. To secure a firm support for 

 the dome, a frame-work of cast iron is constructed below 

 the floor, resting on the walls within the piers. From 

 this frame-work, eight equidistant cast-iron columns ex- 

 tend to the ceiling, and upon these a substantial ring, 

 12 feet in diameter, composed of wood and iron, serves 

 as the foundation for the plate upon which the dome re- 

 volves. 



The western end of the southern pier is extended to 



