394 



where + 5? is the inclination for the year to, x, y, z constants. The 

 annual variation is, 



In all Europe at this time y has a negative value, and z ^positive value ; 

 accordingly the variation is decreasing, and the dip approaching to 



di 



a minimum. Assuming ^=0, and the epoch of the minimum =T, 

 you will find 



As y is tiegative, the last member of T will be positive, and T^-^o- 

 In Siberia y is in the last 10 — 15 years positive, as well as z, and ac- 

 cordingly T-<?o« When y is positive, and z negative, as in Nertchinsk 

 and Pekin in the following table, T is the epoch of a maximum. 



The probable error of the constants a:, y, z, and T, depends on the 

 number and goodness of the observations, the number of years 

 between the first and last, and their more or less symmetrical dis- 

 tribution between these. The most doubtful in the subjoined table 

 are marked with one or two asterisks. 



In examining the table, you will remark that the epoch T of 

 minimum in Europe will arrive earlier in the northern than in the 

 southern places (Paris to Christiania) ; earlier in the eastern than in 

 the western places (Christiania, Stockholm, Petersburg, Kazan, 

 Catharinenburg). For the theory of the magnetism of the earth, I 

 think it of interest to determine this epoch for different points. In 

 order to diminish the uncertainty in different points, I have written 

 to Professor Gedersen in Copenhagen, to Encke in Berlin, to Gauss 

 in Gdttingen, and to KupfFer in Petersburg, and entreated them to 

 make or communicate to me observations nearer to the actual year. 

 And it is the same solicitation I now take the liberty to address to 

 you for London. The only seven observations I know in London are 

 made by Cavendish, 1775 -78 ; Sabine, 1821 '62; Segelike, 1830 -91 ; 

 Lloyd, 1836-50; Phillips, Ross, Johnson and Sabine, 1837-63; 

 Phillips and Fox, 1838 -30 ; Sabine and Ross, 1838 '74. As fifteen 

 years have elapsed since the last observation, and I suppose many 

 .observations have been made in the mean time, I would be extremely 

 obliged to you, if you will be so kind to communicate them to me, 

 or procure a good determination to be made in this year. In 

 reliance upon your interest for this inquiry, to which you have 

 devoted so earnestly your labour and time in a long series of years, 

 I hope you will not be displeased at my entreaty ; and in this hope 

 I remain. Sir, with great respect. 



~=y + 2(t-to)z. 



(II.) 



(III.) 



Your most obedient Servant, 



Chr. Hanstiew. 



