44 



ON THE CONSTRUCTION AND USE OF 



may allow a solar beam three inches in diameter to come in from a heliostat outside. 

 The greatest facilities are thus presented for optical and photographical experiments, 

 for in the latter case the whole room can be used as a camera obscura. 



b. The Dome. 



The roof of the observatory is 20 feet square. The angles are filled in solid, and 

 a circular space 15 feet in diameter is left to be covered by the revolving dome. 

 Although such a construction is architecturally weak and liable to lose its level, yet 

 the great advantages of having the building below square, and the usefulness of 

 the corners, determined its adoption, the disadvantages being overcome by a very 

 light dome. 



The dome is 16 feet in outside diameter, and rises to a height of 5 feet above its 

 base. It is, therefore, much flatter than usual, in fact, might have been absolutely 

 flat, with this method of mounting. It would then have been liable, however, to 

 be crushed in by the deep winter snows. 



It consists of 32 ribs, arcs of a circle, uniting at a common centre above. Each 

 one is formed of two pieces of thin whitewood, b, Fig. 39, fastened side by side, 

 with the best arrangements of the grain for strength. They are three inches wide 

 and one inch thick at the lower end, and taper gradually to 2| by 1. 



Over these ribs tinplate is laid in triangular strips or gores, about 18 inches wide 

 at the base, and 10 feet long. Where the adjacent triangles of tin a a' meet, they 



are not soldered, but are bent together. This allows a certain 

 amount of contraction and expansion, and is water-proof. It 

 strengthens the roof so much, that if the ribs below were taken 

 away, this corrugated though thin dome would probably 

 sustain itself. The tin is fastened to the dome ribs b by 

 extra pieces c inserted in the joint and doubled with the 

 other parts, while below they are nailed to the ribs. In the 

 figure the tin is represented very much thicker than it is in 

 reality. 



This dome, although it has 250 square feet of surface, only 

 weighs 250 pounds. That at the Cambridge (Massachusetts) 

 Observatory, 29| feet in diameter, weighs 28,000 pounds. 



The slit or opening is much shorter than usual, only extending half way from 

 the base towards the summit. It is in reality an inclined window, 2i feet wide at 

 the bottom, 1| wide at the top, and 4 feet long. It is closed by a single shutter, 

 as seen in Fig. 37, and this when opened is sustained in position by an iron rod 

 furnished with a hinge at one end and a hook at the other. 



The principal peculiarity of the dome, the means by which it is rotated, remains 

 to be described. Usually in such structures rollers or cannon balls are placed at 

 intervals under the edge, and by means of rack work, a motion of revolution is 

 slowly accomplished. Here, on the contrary, the whole dome b b' b" (Fig. 40) is 

 supported on an arch h h' 7t", carrying an axis a at its centre, around which a slight 

 direct force, a pull with a single finger, will cause movement, and by a sudden push 

 even a quarter of an entire revolution may be accomplished. It is desirable, how- 



Fig. 39. 



Joints in Tin of Dome. 



