48 THE MODERN REFLECTING TELESCOPE. 



out at one side of the ecj^uatorial head. The tube, cai'iyliig the great weight of the 

 mirror and its cell, is here supported at two opposite sides, instead of from one side 

 only, as in the German forms ; no heavy counterpoises are required ; this form is 

 much better ada[)ted for the coude arrangement of mirrors, so essential in work 

 with very large spectroscopes, only three reflections in all being necessary for this 

 arrangement; furthermore, when the instrument is used at the primary focus, the 

 upper end of the tube is moie easily accessible, in all positions of the instrument, 

 fi-om an observing cai'riage attached to the inside of the dome. 



The weight of the moving parts of the telescope will be about twenty tons. 

 On account of this great weight, and also of the overhang of the fork above the 

 bearings of the j)olar axis, an efficient anti-fi'iction apparatus for the polar axis is 

 demanded, which will at the same time relieve the effect of the overhanging weight 

 of the upper end of the polar axis. The advantages afforded for this purpose by 

 mercury flotation, when this is properly applied, are so great, and the mechanical 

 details for such flotation work out so simply and economically, that this method 

 will undoubtedly be used. 



The proposed mounting will now be briefly described in detail, and attention 

 will be called to many points which are indispensable to the success of a reflecting 

 telescope to be used foi' photography. 



The equatorial head consists of three iron castings, the triangular base-plate m, 

 Plate XI, and the two posts n and o, which carry the bearings for the polar axis. 

 Both posts are hollow, with walls 1|^ inch thick, and are bolted and pinned to the 

 base casting; the post 11 contains the large driving clock. 



The polar axisp is of hydraulic-forged steel, with a head or flange q, 48 inches 

 in diameter and 7 inches thick, forged upon it ; the axis is 14|- feet long over all, is 

 20 inches in diameter for a distance of 2 feet below the head, and is 16 inches in 

 diameter for the remaining 11^ feet of its length; the axis is hollow, with walls 

 4^ inches thick. The bearings of the polar axis are of hard Babbitt metal, and are 

 halved. 



Attached to the lower surface of the 4-foot head of the polar axis is the large 

 hollow disk or float r, 10 feet in diameter and 22|^ inches thick or deep; this is 

 constructed very strongly of angle steel covered with steel plates ^ inch thick ; the 

 whole is finished smooth on the outside, and is turned true in a lathe. The cor- 

 responding trough s is of cast-iron and is turned true on the inside. The inner sur- 

 face of the trough is separated by \ inch all around from the outer surface of the 

 float; this space is filled with mercury. With the dimensions given the immersed 

 part of the float displaces about 45 cubic feet of mercury, which thus floats about 

 nineteen tons, or 95 per cent of the weight of the moving parts of the telescope. The 

 center of flotation is vertically below the center of weight of the moving parts. 

 Oijly three-quarters of a cubic foot of mercury is required to float nineteen tons in 

 this manner. 



The importance in astronomical [)hotography of the smoothness of motion 

 afforded by really efficient flotation of the moving parts cannot be overesvimated. 

 The great size of the worm-wheel t which rotates the polar axis, will mateiially 



