\ siI.vrUED GLASS TELESCOPE. 



31 



the direction of the length of tin- tube \\licii this kiml of support is used, and that 

 is tin 1 reason why the tin edye hoop must terminate in links of chain. 



The interval, eight or ten inches, which separates the face of the mirror from the 

 tube, is occupied by a curtain of black velvet, confined below by a drawing cord 

 and tacked alxne to the tube. This permits access to the mirror to put a glass 

 co\er on it. and \\hen shut down stops the current of air rushing up. "When the 

 instrument is not being used this curtain is left open, because the mirror and tube 

 are in that case kept more uniform in temperature with the surrounding air. 



In spite of such contrivances there is still sometimes a strong residual current 

 in the tube. 1 have tried to overcome it by covering the mouth of the tube with 

 a sheet of Hut glass, but have been obliged to abandon that because the images 

 were injured. At one time, too, \\hen it was supposed that the current was partly 

 from the observer's hod\. heated streams of air going out around the tube, the 

 aperture in the dome was closed by a conical bag of muslin, which fitted the mouth 

 of the telescope tightly. The oul\ ad\antages resulting wore mere bodily comfort 

 and a capability of percehing fainter objects than before, because the" sky-light 

 was shut off. 



b. TJie Supporting Frame. 



The frame which carries the preceding parts is of wood, and rests on a vertical 

 axis (i, Fig. :}(), turning below in a gun-metal cup b, supported by a marble block 



Fig. 30. 



Section of Azimuth Axis. 



resting on the solid rock. The upper end of the axis is sustained by two collars, 

 one c c' above, and the other below an intermediate triangular box e e' from the 

 sides of which three long beams/// 12x3 inches diverge, gradually declining 

 till they meet the solid rock at the limits of the excavation in which the observatory 



