ASTRONOMICAL OBSERVATORIES. 269 



pounds, and the average force necessary to preserve a 

 uniform velocity of rotation is found, by experimenting 

 with spring steelyards, to be about six pounds. 



The equatorial telescope was made by Merz and Sons, 

 of Munich; and has a clear aperture of six inches, with 

 a focal length of eight and a half feet. It has seven 

 negative eye-pieces, magnifying from 36 to 600 times ; 

 a single lens magnifying 940 times ; a prismatic reflector, 

 to which these pieces are all adapted ; and a terrestrial 

 eye -piece, magnifying 80 times. It has two micrometers, 

 one a ring micrometer, the other a filar position microm- 

 eter, having 11 equidistant fixed lines perpendicular to 

 the two movable ones, and may be illuminated in either 

 a dark or bright field at pleasure. It has five positive 

 eye-pieces, magnifying from 128 to 480 times. The hour 

 circle is nine and a half inches in diameter, and the 

 declination circle is thirteen inches in diameter. The 

 telescope is moved in right ascension by an adjustable 

 centrifugal clock. 



The meridian circle, by Simms, of London, is 30 inches 

 in diameter, divided on silver to spaces of five minutes, 

 with two reading microscopes fixed to the piers, and a 

 third placed at a small distance from one of the others, 

 for examining and determining the errors of graduation. 

 The telescope has a clear aperture of four inches, and a 

 focal length of five feet. It has four positive eye-pieces, 

 besides one for collimating by means of a mercurial 

 trough. It is furnished with a declination micrometer 

 and seven equidistant meridian lines. The wires can be 



