HISTORY OF ASTRONOMY. 



rail attached to the base ring of the dome, are placed the 

 six cannon-balls, each six inches in diameter, on which 

 the dome revolves. The dome itself is a complete hemi- 

 sphere, 18 feet in diameter. The opening for the tele- 

 scope is two and a half feet wide, and extends one foot 

 beyond the apex of the dome. The opening for the 



GEOUND PLAN OF DABXMOTJTH COLLEGE OB8EEVATOET. 



telescope is closed by three shutters; the upper one, 

 about 10 feet long, moving on casters over an iron rail- 

 way on the outside of the dome. The two lower ones, 

 about two and a half feet each, rise and fall by means 

 of weights in the manner of common window-sashes. 

 The entire dome is estimated to weigh about 2800 



