32 



ON THE CONSTRUCTION AND USE OF 



is placed. Tliese beams are fastened together by cross-pieces g g g, Fig. 31, and 

 go throngh the floor in spaces h A h, so contrived that the floor does not touch them. 

 At the ends they are cased with a thick leaden sheathing, to deaden vibration 

 and prevent the access of moisture. 



Fig. 31. 



Plan of Observatory (lower floor). 



This tripod support in connection with the sustaining of the telescope by the Avire 

 rope, gives that steadiness which is so essential in photography. Only a slight 

 amount of force, about two pounds, is required to move the instrument in azimuth, 

 though it weighs almost a thousand pomids. 



The plan of the frame centrally carried by the axis a is as follows : From the 

 corners of a parallelogram ii (2x13 feet) of wooden beams, eight inches thick 

 and three inches broad, perpendiculars n n' , Fig. 28, rise. At the top they are 

 connected by lighter pieces to form a parallelogram, similar to that below, and just 



