8 



Mr. Balfour Stewart on the 



[Jan. 25, 



January 25, 1866. 



Lieutenant- General SABINE, President; in the Chair. 



In accordance with the announcement made from the Chair at the last 

 Meeting, the President read letters from Dr. William Bird Herapath, ex- 

 plaining the reason for non-payment of his annual contribution ; and the 

 question of his readmission was put to the vote, and was decided in 

 the affirmative. The President accordingly declared that Dr. Herapath 

 was readmitted into the Society. 



The following communication was read : — 



" Note on the Secular Change of Magnetic Dip, as recorded at 

 the Kew Observatory." By Balfour Stewart, M.A., LL.D., 

 F.B.S., Superintendent of the Observatory. Received January 

 10, 1866. 



The President of this Society has already called the attention of the 

 Fellows to the annual values of the magnetic inclination at Toronto, as de- 

 duced from the monthly determinations. In doing so he remarked that 

 " the general effect of the disturbances of the inclination at Toronto is to 

 increase what would otherwise be the amount of that element ; therefore, 

 if the disturbances have a decennial period, the absolute values of the in- 

 clination (if observed with sufficient delicacy) ought to show in their 

 annual means a corresponding decennial variation, of which the minimum 

 should coincide with the year of minimum disturbance, and the maximum 

 with the year of maximum disturbance." At Toronto, where the true 

 secular change is very small, the effect of this superimposed variation is 

 very visible, so that the yearly values of the inclination appear to increase 

 up to the period of maximum disturbance and to decrease after it. At Kew 

 the general effect of disturbances is probably the same as at Toronto — that 

 is to say, tending to increase the inclination ; but the secular change being 

 considerable, and tending to decrease the inclination, the joint effect of the 

 secular change and the superposed variation might be expected to appear 

 in a diminution of the yearly secular change for those years during which 

 the disturbances are increasing from their minimum to their maximum 

 value, and in an increase of the yearly secular change for those years during 

 which the disturbances are decreasing from their maximum to their 

 minimum. 



The Kew records appear to exhibit a variation of this nature. Observa- 

 tions of dip were commenced at the Kew Observatory in 1854 ; and by 

 comparing a good number of observations taken during the latter months 

 of 1854, with two circles and four needles, with observations taken with the 

 same circles and needles during the same months of 1855, we obtain a yearly 

 secular change of 2'* 24. 



