68 Rev. W. Sidgreaves Magnetical Observations. [Feb. 9, 



Note by the President. 



Mr. Sidgreaves's observations, combined with those made at the same spot 

 in 1858 in the course of the second magnetic survey of England, supply 

 the materials for a first approximate deduction of the present amount of the 

 secular change of magnetic dip and of the total magnetic Force at Stony- 

 hurst. 



Commencing with the Dip : the results in 1858 were as follows (Brit. 

 Assoc. Reports, 1861, pp. 253 & 254) : 



Sept. 20. Kew Circle No. 30. Needle 1 .... 70 012 Rev. W. Kay. 

 Nov. 2. Kew Circle No. 32. Needle 1 .... 69 57 44 Rev. A. Weld. 



14. .... 70 3 30 



14. ....70 4 21 

 Mean : corresponding in date to 1858.8 70 1 27 



And by the present Observations, correspond- 

 ing in date to 1864.5 69 46 34 



Difference, corresponding to 5.7 years .... 14 53 



whence we have an annual secular decrease of 2'-614 ; mean epoch 1861.9. 



In a memoir presented to the Royal Society in 1861, " On the Secular 

 Change of the Dip in London between 1821 and 1860," printed in vol. xi. 

 of the 'Proceedings,' pp. 144-162, the mean annual secular decrease of 

 the dip in the years from 1821.65 to 1859.5 is stated to have been 2'-69, 

 mean epoch 1840.6 ; and in the 21.2 years between 1838.3 and 1859.5, 

 2-63 ; mean epoch 1848.9. 



Proceeding to the Total Force : its value obtained by myself at Stony- 

 hurst by experiments of deflection and vibration with the Survey Collimator 

 No. 5, in October 1858 was 10'385 in British units (Brit. Assoc. Reports, 

 1861, pp. 264, 268) ; and by the experiments of Mr. Sidgreaves with the 

 apparatus belonging to Stonyhurst College (originally obtained from Kew), 

 its mean value in 1864, derived from the twelve monthly determinations, was 

 10'4031 ; the difference is '0181 in 5.75 years, or an annual increase of 

 0031. To compare with this, we have the statement in the British Survey 

 (Brit. Assoc. Reports, 1861, p. 273), that from the absolute measures 

 made monthly at Kew between April 1857 and March 1862 the total force 

 had increased at Kew during that interval at an average annual rate of 

 0025. In the same memoir it was also inferred, from a general comparison 

 of the isodynamic lines in the first and second British Surveys, that along a 

 line drawn in a N.W. and S.E. direction the secular change would be found 

 contemporaneously somewhat greater at a northern or north-western station 

 than at a southern or south-eastern station greater therefore at Stonyhurst 

 than at Kew. The general fact that the value of the total force in Britain 

 is progressively increasing, may be inferred alike by the observations at Kew 

 and at Stonyhurst ; the precise amount of the annual increase at either 

 station will require a longer continuance of the same careful and systematic 

 observations as those at Kew and Stonyhurst. 



