﻿DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 185 



Spider-line Micrometers. 



The position-circle of the micrometer at the divisions is seven inches in diameter, 

 reading by two verniers to single minutes. The head of the micrometer-screw is divided 

 into one hundred parts, each being of the value of one tenth of a second of arc nearly. 

 The equality of the values of the revolutions and half-revolutions of the screw has been 

 examined over a space of fifty revolutions by transits of Polaris. 



Revolutions in different parts of the screw are found to be sensibly equal, and no 

 irregularities were detected beyond the limits of errors to which the determination was 

 liable. 



The values of a revolution of the screw for different temperatures are as follows : — 



The dilatation of the screw seems to have greater effect on the value of r than the 

 increase of focal distance, arising from the expansion of the object-glass. 



The workmanship of the machinery of the mounting is creditable to the makers, and 

 the general plan combines strength with simplicitv and convenience of management. 

 The friction of the various actions is greatly relieved by the judicious arrangement of 

 counterpoises and friction-wheels, so that the whole is moved with great ease by a single 

 finger, though weighing about three tons. 



It has been objected to the German form of mounting a telescope equatorially, that it 

 requires reversal whenever the object under examination crosses the meridian. This is 

 felt as a practical inconvenience with the Cambridge equatorial only in small zenith dis- 

 tances, since in most instances the telescope passes the meridian by more than an hour 

 of right ascension, and always by more than two hours in southern declinations. 



