SECULAR CHANGE. 



81 



14. A comparison of the mean values of an element for corresponding months of consecutive years 

 enables an estimate to be formed of the rate of secular change. 



Table VI shows the results thus found for 1) and II, accepting the monthly mean values given in 

 Tables IV and V for the five months May to September. The data subsequent to September, 1903, wore 

 too few to give representative results, while during March and April, 1902, matters were still somewhat 

 in a preliminary stage. 



TABLE VI. Secular Change. 



The results for the secular change in H show a rather unexpected consistency. The only suspicious 

 feature in the figures, as figures, is the extraordinarily large size of the apparent annual change, 

 representing as it does nearly 2 per cent, of the absolute value of the Horizontal Force. If the mean is 

 a true measure of the secular change, the natural inference is that the south magnetic Pole is receding from 

 Winter Quarters i.e., is moving northwards at a rapid rate. 



The Declination figures are no less remarkable, but appear much less consistent. Starting with an 

 apparent decrease of 103'-! in the year ending with May, 1903, we finish with an apparent increase of 

 8' 7 in the year ending with September, 1 903. 



Such a phenomenon seems hardly credible, and one cannot but suspect some instrumental source of 

 error. In the case of the Declination, as already stated, there is no apparent reason why the base line 

 should change, and it is conceivable that some seasonal change may have influenced the absolute 

 observations, especially in view of the apparent inconsistencies in the azimuth readings obtained for the 

 distant mark ("Physical Observations," p. 139). I have thus thought it worth while to ascertain what 

 results would be obtained for the secular change of Declination if the base-line value were assumed to be 

 invariable. On this hypothesis the results given in Table VI are replaced by those in the following 

 Table VII. 



TABLE VII. Secular Change of Declination. 



The results given in Table VII are the antithesis of those in Table VI, and are more consistent amongst 

 themselves. Whilst Table VI suggests that the south magnetic Pole is moving towards the west, 

 Table VII suggests that it is moving towards the east.* 



15. In even the best European stations two years is too short a period to give results of a really repre- 

 sentative character for the annual inequality of the magnetic elements, i.e. the variation that remains in the 



* The results obtained for the secular change by Commander CHETWYND, R.N., from the absolute observation! alone, 

 disregarding diurnal variation, were for Declination 26' '4, for Horizontal Force + 130y ("Physical Observations," 

 pp. 137, 140). 



M 



