Horizontal Component of the Earth's Magnetic Field. 487 



can be readily made by turning the screw s until equal deflec- 

 tions are obtained on opposite sides of zero, when the deflector 

 is reversed by turning the plate g through 180° in azimuth. 

 The deflectors, rf, are shown in positions east and west of 

 the magnetometers ; and the distance between their centres 

 when in this position is 70 centim. Two plates similar to p 

 are placed, one on the north and one on the south side of the 

 magnetometer, in such a position that, when the deflector-stand 

 is resting on them, the distance between the centres of the 

 deflectors is 60 centim., and the line joining them is in the 

 magnetic meridian. The feet on which the deflector-stands 

 rest are so adjusted that the deflectors are equidistant from the 

 magnetometers, and at the same distance apart when the posi- 

 tions of the stands are interchanged. The scale C is graduated 

 to millimetres on glass, and is placed with its centre directly 

 in front, and in the focus, of the magnetometer-mirror. The 

 distance between the scale and the mirror is 129 centim. 



A paraffin-lamp, with a copper funnel which has a vertical 

 slit in front, with a fine wire in its centre, covered by a plane 

 glass plate, is placed behind the scale, and furnishes a beam of 

 light which is focused on the glass scale by the magnetometer- 

 mirror. The scale being transparent, the deflections can be 

 read either from the back or the front of it. When the deflec- 

 tions were read from behind the scale, it was found convenient, 

 although not absolutely necessary, to render the glass partially 

 obscure ; and this, acting on a suggestion of Mr. Bottomley, 

 was done by dusting lycopodium -powder over it. The deflec- 

 tions were taken in the following manner : — 



The table T was first placed in such a position that the line 

 joining the centres of A and B (PI. XI. fig. 1) was exactly at 

 right angles to the magnetic meridian. This was done in one or 

 other of the following ways : — (1)A thin wire was passed under 

 the magnetometer and stretched along the line joining the 

 centres of A and B, and then taken back, either over the top of 

 the magnetometer, or beneath and at a greater distance from it, 

 in such a way as to form a vertical plane circuit. An electric 

 current was then sent through the circuit, and the table turned 

 until it produced no deflection on the magnetometer-needle. 

 (2) One of the deflectors was placed in its position north or 

 south of the magnetometer, and lifted out of its V by the sus- 

 pension-fibre. The table was then turned until the suspended 

 needle produced no deflection of the magnetometer-needle. 

 When this is the case, the direction of the magnetic axes of 

 the deflector and the magnetometer-needle are in the same line ; 

 and if the latter needle be in its proper position, this line also 

 passes through the centre of the deflector when placed on the 



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