72 



MR BROUN ON THE BALANCE MAGNETOMETER, 



tion. The following series of observations made during a magnetic disturbance, 

 will, however, prove it more difstinctly. 



Table IV. 



Observations for the Time of Vibration of the Balance Needle in a Vertical 

 Plane, the position of rest varj'ing. 



The positive and negative signs indicate that the north pole of the needle 

 was below or above the horizontal. It would have required a change of 50° Fahr. 

 to have produced alone a difference of 400 micrometer divisions. Such a change 

 of temperature, according to f 6, would have been equivalent to a change of 



3.8 in the time of vibration. The observed times differ only a few tenths, and the 

 times corrected for temperature agree within the limit of the eiTors of observa- 

 tion.* 



9. It results from these facts, that the time of vibration in a vertical plane 

 cannot be used at present in the reduction of the observations, as theory takes 

 no account of them. Tiie theoretical corrections for differences of arc or the varia- 

 tion of the moment of inertia due to temperatiu-e would, in the examples given, 

 be inappreciable. 



10. I shall now consider — -, the temperature correction for the position of 



the needle. 



The method which has been adopted for its determination is as follows : — 

 The magnet, whose temperature coirection is to be obtained, is placed at right 

 angles to a magnet freely suspended, which is thus deflected by an angle u from 

 the magnetic meridian. If ni be the magnetic moment of the deflecting magnet, 



* The time of vibration througliout the year varies from other causes. The law which regulates 

 these variations I have not yet determined. 



