238 



HISTORY OF ASTRONOMY. 



foundations of the building were laid, and it was fin- 

 ished in the spring of 1844. 



The ground on which the observatory is built is 154 

 feet above the level of the Potomac river, from which 

 it is distant about half a mile. The central part of the 

 building is 30 feet square on the outside, with connecting 

 wings both on the east and west sides, each of them 

 being 27 feet by 15, making the entire length of the 

 observatory 60 feet. The central part is surmounted by 

 a rotary dome 20 feet in diameter, which works on 

 cast-iron rollers, 8 inches in diameter. The opening in 



GEOBQETOWN OBBEBVATOEY, SOTJTH ELEVATION. 



fl 



the dome is 2 feet wide, and is closed by 4 shutters. 

 From the cellar, through all the floors of this part of 

 the building, rises a pier of masonry 41 feet high. This 

 pier is 11 feet square at the base, and 6 feet square at 



