434 



Introduction to the Sixth Section 



exactly in the same straight line: the points of the pivots 

 which touch theY's are distant from those, on which the level is 

 suspended 9 lines ; an angle of 9" of the axes of the two cylin- 

 ders would therefore account for the small deviation. If this 

 be really the cause, the repetition of the levelling in reversed 

 positions of the telescope entirely destroys its effect. The 

 diameters of the two pivots are not exactly equal, the one 

 more distant from the circle is the thicker. Three-and-thirty 

 reversions of the instrument have given the difference of the 

 level in both positions =1*286 line: viz. 



The hooks by which the level is hung to the pivots, form 

 an angle of 90°, the Y's an angle of 60°. If the diameter of 

 the thicker cylinder be denoted by ?•', that of the thinner one 

 by /-, the height of the points in which the planes forming the 

 Y's meet above the same level, that of the eastern one by //, 

 that of the western one by h' ; the heights of the centres of 

 the axis, when the circle is towards the east, are 



h + 2r, and h' + 2r' 

 and the heights of the points where the planes forming the hooks 

 of the level meet h + r (2 + V 2) and h' + r (2 -f. V 2). 

 The motion of the bubble to the west is therefore, in lines (if 

 R expresses the length of the axis) 



= x = y-*)+y-;yaw«) and after reversin 



R sin 2"- 164 fc 



«■' =s 



