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 f;icilities 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 s(|uar('. 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 luiAing 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 dinmeter, and rises to a height of 5 feet above its 

 base. It is, therefore, much Hatter tb;ni usual, in fact, might have been absolutely 

 flat, Avitli 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 (if thin whitew'ood, ?*, Fig. 39, fastened side by side, 

 with the best arrangements of the grain for strength. Tlu^y are three inches wide 

 and one inch thick at the lower end, and taper gradually to 'l\ by 1 . 



O^er tlu^se ril)s tinplate is luid in triangular strips or gores, about IS inches wide 

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



are not soldered, but are bent together. Tin's aUowsa certain 

 amount of contraction and expansion, and is water-])roof. It 

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

 away, this corrugated though thin dome would ])rol)ably 

 sust;iin itself. 'I'he tin is fastened to the dome ribs /> by 

 extra pieces c inserted in the joint and (loul)led with the 

 other ])arts, while Ixdow they are nailed to the ril)s. In the 

 ftgure the tin is represented very much thicker tlian it is in 

 reality. 



'I'his dome, although it has 250 square feet of surface, only 

 weighs 250 pounds. That at the Cam])ridge (Massachusetts) 

 Observatory, 293 fi'ct in diam(^t(n', weighs 28,000 pounds. 



The slit or opening is nuich shorter than usual, only extending Iwilf way from 

 the base towards the summit. It is in reality ;in inclined window, 22 feet wide at 

 the bottom, 1| wide at the top, and 4 feet long. It is closed l)y a shigle 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 whicli it is rotated, r(>mains 

 to be described. Usually in sucli structiu-es 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 I h' h" (Fig. 40) is 

 supported on an arch li li' h", 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 (piarter of an entire n'\olution may be accomplished. It is desirable, how- 



Fig. 39. 



' ',-M:.„„^^,/X!S7r/f. 



tr 



J 



/^•-v, 



.Toiiits in Tin of I-)t)nie 



