274 HISTORY OF ASTRONOMY. 



The central pier is built of brick, and is 8 feet square 

 through the first story, which is 15 feet high, but through 

 the second story is only 6 feet square. This pier is sur- 

 mounted by a traveling dome, 17 feet in diameteif and is 

 pierced by an opening 2 feet wide, extending from a 

 point 4 feet above the floor to 8 inches on the opposite 

 side of the zenith. The dome rests on twelve eighteen- 

 pound balls, which turn between two cast-iron annular 

 grooves. 



The transit pier is five feet square, and extends three 

 feet into the second story. The meridian observing slits 

 are made two feet in the clear, and afford an uninter- 

 rupted view of the meridian from the northern to the 

 southern point of the horizon. Both piers are covered 

 with marble slabs, three inches thick. 



The equatorial telescope, which is erected under the 

 dome, was made by Henry Fitz, of New York. It is a 

 refractor of 11 feet focus and 9 inches aperture ; and has 

 a position micrometer furnished with an illuminating ap- 

 paratus. The hour circle is 12 inches in diameter, and 

 the declination circle 15 inches, each reading by two 

 verniers. The telescope is moved by clock-work, so 

 that the object under examination may be kept steadily 

 in the center of the field. This telescope is furnished 

 with ten eye-pieces, the highest magnifying 800 times. 



The transit instrument is in the second story, has a 

 clear aperture of 2 inches, and a focal length of 33 

 inches. Near the transit is the clock with a mercurial 

 pendulum. 



