244 



A Cheap Observatory. 



broken and fragmentary, were all the more suitable for turning 

 the sharp curve. The walls are nine inches thick throughout, 

 and the interior diameter of the enclosed space nine feet. Two 

 courses from the top, and at equal intervals, are inserted six 

 slabs of stone, to which are securely bolted the cast-iron chairs 

 carrying the flanged wheels, on which the roof was intended 

 to revolve. The wheel and its axle, and the chair, were cast in 

 separate pieces, and required, therefore, only two very simple 

 patterns. The wheels required turning in a lathe to render 

 them true, but the chairs were trimmed up and finished with 

 a file, and the whole when completed cost exactly 22s. In 

 setting the wheels great care was bestowed to range them 

 accurately in a circle, and to ensure this each one as set was 

 tested by a wooden radius working on a firm support at the 

 centre. They were also accurately levelled, the one from the 

 other, and when finished, the upper bearing edge stood half an 

 inch above the level of the final ring of brickwork. 



The diameter of that part of the wheel which carries the 

 weight is four inches. The flange extends beyond this three- 

 quarters of an inch more, and the surface of the bearing part 

 is one inch wide, which allows for slight irregularity in the iron 

 circle. It might also be mentioned that in order to do away 

 with friction, the flange is not perpendicular to the bearing 

 surface, but reclines away from it, hence the edgo of the iron 

 ring comes in contact with the flange only at its base. The 

 wheels may appear rather small, but they are found to answer 

 most perfectly, and the roof moves with freedom. Out of the 



six wheels it rarely happens that 

 more than three take a bearing at 

 one time, but when one leaves off 

 another begins. 



A sketch of their appearance 

 when in situ before the roof was 

 put on is here given. We next 

 coino to the wooden part of the 

 building, namely the roof. Hero 

 again economy interposed and for- 

 bad all the woodwork being planed, 

 so it was used up simply as it came 

 from the saw. 



The framework of tho roof is 

 made up of two circles, four verti- 

 cal standards, and two cress beams. Tin; circles are both 



of elm, and were ciil in segments from boards one-and-a-half 

 inches thick, they were placed end to end on a level floor and 

 united by other Begments only an inch thick, theso were laid 

 across the joints, and all finnJy united by screws. 



