TERRESTRIAL MAGNETISM 305 



axis being level with the centre of the scale. The needle when in 

 position moves in a plane midway between the agate knife edges. 



A cross arm provided with verniers and two microscopes 

 having cross wires in their fields of view can be turned round till, 

 say, the upper end of the needle is on the cross wire of one 

 microscope and the upper vernier is read. The cross wire in the 



FIG. 226. 

 Diagrammatic representation of the Dip Circle. 



other microscope is then brought to coincide with the lower end 

 and the lower vernier is read. The mean reading gives the 

 position of the needle on the scale. 



When an observation is to be made, the axis round which the 

 dip circle rotates is made vertical and the needle is put on to 

 V- bearings, which are then lowered so as to leave it on the knife edges. 

 We shall suppose that the dip circle is already adjusted in the plane 

 of the meridian, and that the face of the instrument is East. The 

 position of each end of the needle is then read. The mean of 

 these eliminates error due to a small error in the centering of the 

 needle. For if NS, Fig. 227, is the line through the ends of the 

 needle and it does not pass through the axis of the circle, draw N'S 

 parallel to NS, and HOH' the horizontal through (). Then 

 SON' = SOS', and HON + H'OS = HON' -- NON' + H'OS' -f- 

 >()>' = I ION' + H'OS', or the mean of the readings for N and S 

 gives us the mean of the readings for N' and S'. 



The vertical circle may not be correctly set that is, the points 

 H and 1 1' may not be at on the scale. The circle with the 

 needle on it is therefore turned through 180 round the vertical 



so as to face West, and the positions of the ends of the needle 



are again read. 



u 



