J. Campbell's Astronomical Observatory. 65 
side, with four iron bands at equal distances, to strengthen the 
structure, and keep the ribs in their proper positions, as they 
are what may be termed the rad/road of the door which closes 
the aperture. The dome is covered with tin, except the space 
between the two main ribs; on one side this space is covered 
ment of the chain or guide pullies, which this zinc covering 
overlies. 
4. 
# 
A, Wooden curb or bed plate.—B, Circular rail—C, Base of the dome.—D, Rack 
encircling the rail.—E, ‘The pinion. 
