AT STATIONS ON THE COASTS OF THE BEITISH ISLES, 1907. 243 



17. The mean annual change at Kew during the 16-year period, 1891 to 1907 , is 

 4 /- 9. Comparing this with the values at those stations at which the 16 -years' interval 

 is available, viz., Great Grimsby, Milford, Suriderland, Letterkenny, Loch Melfort, 

 Stonehaven, and Tanera Mor, the mean of the values at the Field stations is /- 2 

 greater than at Kew. 



At Milford, where results are available for both the 21- and the 16-year periods, 

 the mean for the former is 0' - 5 greater than Kew, and for the 16-year period is the 

 same as Kew. 



18. These results seem to indicate that the reduction in the amount of the annual 

 change has been greater at the Field stations than at Kew. 



19. This is corroborated by a comparison (p. 244) of the Declination-change curves 

 for Kew and Stonylmrst (fig. I). 



20. Comparison of Mean Annual Chanc/e of Declination at Kew, Greenwich, 



and Stonyhurst. 



To compare the annual change at Kew, Greenwich, and Stonyhurst, the mean of 

 five successive yearly differences of Declination is assigned as the mean annual change 

 for the mid-year of the 5-year period. 



Thus the mean of the yearly differences from 188G. to 1891.,-, is assigned as the 

 mean annual change for the year 1889. The mean annual value representing the 

 value for the middle of the year, the quinquennial mean change is allotted to 

 1st January of the middle year. 



21. Curves of which these quinquennial mean values are the ordinates have been 

 drawn, as shown in fig. 1. 



22. The comparison of these curves indicates that between the years 1886 and 1894 

 (embracing the period of HUCKER, and THORPE'S survey) the value of the secular 

 change at Stonyhurst Avas considerably in excess of that at Ke\v this being in 

 agreement with the results found by RUCKER and THORPE that the value was greater 

 in the North-west than at Kew. 



Since the year 1894 the values at Stonyhurst and Kew are in closer agreement and 

 that at Stonyhurst slightly less than at Kew. 



23. Figs. 2, 3, and 4 show respectively the curves of annual change of Declination, 

 Horizontal Force, and Inclination at Kew derived from quinquennial means. 



24. The Declination-change curve, fig. 2, indicates that the annual change which 

 was decreasing in amount from the epoch 1893 to 1904 is now increasing in amount, 

 and that the mean value at the present epoch (l January, 1907) is 4''8. 



25. Assuming the mean difference between Kew and the rest of the British Isles, 

 derived from observations at 16 years' interval, to be as given (see paragraph 17), 

 the mean value of the change for the United Kingdom is 5'. 



26. The Horizontal Force-change curve, fig. 3, indicates that the amount of annual 



2 I 2 



