32 



ON THE CONSTRUCTION AND USE OF 



is placed. Tlicsc beams are fastened togetlier by rross-pieces [/ f/ g. Fig. 31, and 

 go through tlie floor in spares 7i h h, so contrived that the floor does not toucli them. 

 At tlie ends tliey ar(^ (as(Hl \vitli a tliick leaden slieathing, to deaden vibration 

 and prevent tlie access of moisture. 



Fis. 31. 



Plan of Observatory (lower floor). 



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

 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 pounds. 



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

 corners of a parallelogram // (2 < 13 feet) of wooden beams, eight inches thick 

 and three inches broad, perpcndicidars n n\ Fig. 28, rise. At the top they are 

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



