Rocks and Regional Magnetic Disturbances. 509 



distances from it, it was found necessary to introduce another small 

 compensating solenoid (C) which could be so adjusted as to neutralise 

 the residual effect. 



The mirror was raised sufficiently above the horizontal plane 

 through the axes of the two solenoids to permit a beam of light being 

 thrown upon it and reflected to a scale. An experiment was per- 

 formed as follows : 



The small solenoid circuit s was first thrown out of connexion, and 

 the main circuit, together with the shunt B, put in. The zero read- 

 ing, as given by the magnetometer, having been taken, the apparatus 

 was so arranged that the zero did not alter when the current was put 

 on, or off, or reversed. The tube was now inserted into each of the 

 solenoids in turn, the deflections were noted, and afterwards the zero 

 was again taken to show that no change had occurred during the 

 experiment. The shunt B was then thrown out, and the circuit con- 

 taining the small solenoid s put in, the resistance in the Post Office 

 bridge having been previously arranged so as to give a deflection of 

 much the same magnitude as that due to the introduction of the tube 

 containing the mixture of magnetic oxide and glycerine. Knowing 

 the deflection which is given by a solenoid of known moment, and 

 that produced by the introduction of a definite amount of mixture 

 under the same conditions as regards the sensibility of the magneto- 

 meter, we are able to calculate the susceptibility of the mixture, as 

 follows : 



Let 26 be the length of the tube. 



f, i], the co-ordinates of the centre of the magnet referred 

 to three rectangular axes, passing through the 

 middle point of the axis of the tube, parallel to its 

 length, and perpendicular to its length in the hori- 

 zontal and vertical directions respectively. 



a, ffj, the distances from the centre of the magnet of the feet 

 of the perpendiculars let fall from the ends of the 

 tube on the horizontal plane through the magnet. 



It is then easy to show that if the length of the magnet is small as 

 compared with <rand o', and if p be the strength of the pole induced at 

 the end of the tube, 



where F is the strength of the field in which the magnet is placed, 



d the deflection produced by the introduction of the tube 

 containing the liquid into the solenoid, 



D the distance of the scale from the magnet, 



