214 Mr. Samuel Okell on a 



of strong sail canvas stretched over a wooden framework, 

 and may best be described as consisting of three fiat zones, 

 which form chords to the great circle. The main ribs run, 

 that is half of them, from a semicircular frame at the top 

 to the circular wooden ring which forms the base of the 

 dome, the two horns of the semicircular frame being 

 carried down parallel to form the opening, four feet wide, for 

 observing purposes. The remaining half of the radial ribs 

 run into, and are joined into, the sides of this opening. 

 There are smaller cross ribs placed at intervals of 15 to 18 

 inches, both for the purpose of stiffening the whole structure 

 and also to serve as further supports for the canvas. To 

 add still more to the general strength, two strong iron stays 

 are adapted to the shape internally, and run from the base 

 to the middle of the side of the opening, one on each side of 

 the observing slit at about an angle of 45° The rail on which 

 the dome revolves is fixed to the underside of the wooden 

 circular base of the dome, the whole structure running round 

 on six wheels bolted on to timbers built into the main circular 

 brick walls of the observatory. The spindles of two of the 

 wheels, one on each side of the house, are lengthened, and 

 have hand wheels 2ft. 6in. diameter attached, by which the 

 dome is very easily revolved. It will be noted that, the rail 

 being attached to the dome, the wheels are stationary, the 

 dome revolving over them ; one advantage of this arrange- 

 ment being, that the hand wheels for revolving the dome 

 are always in the same position with respect to the walls, 

 and may be found in dim light without difficulty, and 

 another, that the walls are continued up close to the base of 

 the dome ; narrow oilcloth curtains attached to the inside and 

 outside edges of which effectually cover the joining and keep 

 out draughts. The shutter, which is also of canvas, is made 

 on a framework of cross ribs of wood, with small wheels 

 and clips at either end, the ribs being about 20 inches apart, 

 and is stayed by strips of broad spring steel, which adapt 



