TERRESTRIAL MAGNETISM. 705 



This galvanometer was found to be too delicate, and only one 

 needle was afterwards used for the observations which were made 

 on Jan. 7, 11, 15, 16, and June 25, 26, 27, 28. 



The mean value of the dip from these determinations is 

 790 42/ 4//. 



Hourly observations for changes of dip were also made with 

 the dipping needle at 4 minutes past the hour on all term days. 



On Feb. 11, from 6h. to 12h., there was found to be first a 

 great increase, and then a great diminution in the dip, accom- 

 panied by great oscillations. The change of dip in 2 hours 

 amounted to about 2°. 



On March 21, especially at 18h. and 19h., there is great dimi- 

 nution in the dip. 



[On comparing the tables it seems that magnetic disturbances 

 which cause an increase in the readinoj of the scale of the maojneto- 

 meter also cause a diminution in the dip, and those which 

 diminish the reading of the scale increase the dip. 



This is true of most of the simultaneous disturbances observed 

 on the term days, February 11 and March 21. 



Since an increase in the reading on the scale of the magneto- 

 meter means a diminution in the westerly declination, we see, by 

 comparing the observations for declination and dip, that a disturb- 

 ance which causes an increase 171 the westerly declination also 

 causes an increase in the dip?^ 



12. Method of Obseevation with the Induction Magnetometek. 



A circular coil of copper wire, to the ends of which the gal- 

 vanometer wires are attached, is capable of rotating through 180° 

 about an axis which may be placed in a horizontal or in a vertical 

 position in the magnetic meridian. When the coil is rotated at a 

 given rate, it cuts the lines of terrestrial magnetic force, and a 

 current is induced in the wire, the strength of the current being 

 proportional to the number of lines of force cut during the half 

 revolution. The number of lines of force cut by the revolving 

 coil will depend on the angle between their direction {i.e., the 

 direction of the dip) and the axis of the coil, being proportional 

 to the sine of half the angle. 



If ^ and ^1 be the deflections of the galvanometer when the axis 

 is horizontal and vertical respectively, and ^ the dip, then 



9 



tan I = 



sm 

 2 



smiii 

 2 



If X be the observed reading on a reflecting galvanometer, when 

 the scale is at a distance r from the centre, then 



tan 2 (3 =1. 



3G122. y Y 



