MANIPULATION OF THE DIPPING-NEEDLE. 81 



can rotate round an axis, which must be made accurately 

 vertical. The divisions and 180 of the circle as read 

 by the microscopes ought then to correspond to vertical 

 position of the needle. In general, it is sufficient to 

 trust to the artist for this adjustment : but at the Royal 

 Observatory, Greenwich, a loaded brass needle has been 

 introduced, whose position is read in the same manner 

 as that of the magnetic needles ; and by use of this the 

 accuracy of the divisions and 180 can be verified. 



37. Manipulation of the Dipping -Needle : reversion 

 of its pivot-bearing : rotation round its vertical axis : re- 

 versal of its magnetic poles. 



The point to which the theoretical considerations, 

 employed in the use of the dipping-needle, are particu- 

 larly addressed, is the elimination of errors produced by 

 the non-coincidence of the axis of rotation of the needle 

 with its center of gravity. Measuring from the axis 

 of rotation, the center of gravity may have an ordinate 

 of sensible value in the direction longitudinal to the 

 needle, and one in the direction transversal to the 

 needle. It will be easily conceived that the effect of 

 the latter ordinate may be eliminated by reversing the 

 pivots upon the agate edges, so as to present that edge 

 upwards which was downwards (see the difference be- 

 tween Figures 31 and 32, or between Figures 33 and 34). 

 The same effect may be produced by rotating the entire 

 supporting frame with the needle which it carries, round 

 a vertical axis, through 180 : for, as the vertical force 

 of magnetism tends to depress the same end as before, 



6 



