MAGNETIC SURVEY OF JAPAN. 1^7 



Dient of the azimuth of the coil. If the current is now broken and 

 the scale image does not shift, it is certain that the magnetic axis of 

 the coil lies in the magnetic meridian. The reading on the theodolite 

 gives its azimuth. 



During the series of operations, however, the magnetic meridian 

 might have changed — during the forenoon, indeed, the change is 

 quite perceptible in five minutes. Or, the mngnetometer stage might 

 itself have got shifted somewhat in the process of adjustment. In 

 either of these cases there will be a shifting in the scale image at the 

 instant of making the final break in the circuit. By adjusting, how- 

 ever, until at the final break the scale image remains absolutely 

 unaffected, we know that at that instant, which should be noted on 

 the chronometer, the magnetic meridian is determined. An exactly 

 similar observaticjn must be made with the coil reversed as regards 

 east and west ; and then the mean of the two readings so obtained 

 may be assumed to give the declination at that instant corresponding 

 to the mean of the two instants of adjustment, quite independent of 

 any error resulting from any (slight) deviation from perpendicularity 

 between the axis of the pivots and the line of magnetic force at the 

 centre of the coil due to the current in it. An experienced hand can 

 make the interval between the two final adjustments as small as 2|- 

 minutes. 



For the determination of the horizontal force, the declinometer 

 coil must be removed and in its place a deflection bar substituted as 

 show in fiuure 5, Plate VI. This bar is made of brass and has a 

 Y-groove on its upper surface — or rather two A'-grooves extending 

 the one to the east and the other to the west, when the instrument is 

 mounted ready for use. AYhere the bar rests on the Y's, it is made 

 in the form of a semi-cylinder — the upper surface being flat, the 

 lower having the same curvature as the pivots of the theodolite tele- 



