MAGNETIC INCLINATION. 109 



inclination by eight hours, while the maximum of intensity 

 differs only by two hours from the minimum of inclination.* 



The periodicity of inclination at the Cape of Good Hope 

 does not coincide with that at Hobarton, which lies in the 

 same hemisphere, nor with any one point of the northern 

 hemisphere. The minimum of inclination is indeed reached 

 at an hour at which the needle at Hobarton has very nearly 

 reached the maximum. 



For the determination of the secular variation of the in- 

 clination it is necessary to have a series of observations that 

 have not only been conducted with extreme accuracy, but 

 which have likewise extended over long intervals of time. 

 Thus, for instance, we can not go with certainty as far back 

 as the time of Cook's voyages, for although in his third ex- 

 pedition the poles were always reversed, we frequently ob- 

 serve differences of 40' to 55 ' in the observations of this 

 great navigator and of Bayley on the Pacific Ocean, a dis- 

 crepancy which may very probably be referred to the imper- 

 fect construction of the magnetic needle at that time, and to 

 the obstacles which then prevented its free motion. For 

 London we scarcely like to go further back than Sabine's 

 observation of August, 1821, which, compared with the ad- 

 mirable determination made by himself, Sir James Ross and 

 Fox in May, 1838, yielded an annual decrease of 2 /- 73, 

 while Lloyd with equally accurate instruments, but in a 

 shorter interval of time, obtained at Dublin the very accord- 

 ant result of 2 x> 38.f At Paris, where the annual diminution 

 of inclination is likewise on the decrease, this diminution is 

 greater than in London. The very ingenious methods sug- 

 gested by Coulomb for determining the dip had, indeed, led 

 their inventor to incorrect results. The first observation 

 which was made with one of Le Noir's perfect instruments 

 at the Paris Observatory belongs to the year 1798. At that 

 time I found, after often repeating the experiments conjoint- 

 ly with the Chevalier Borda, 69 5T; in the year 1810, in 

 conjunction with Arago, I found 68 50 /- 2 ; and in the year 

 1826, with Mathieu, 67 56'-7. In the year 1841 Arago 

 found 67 9 X , and in the year 1851 Laugier and Mauvais 



* Total intensity at Hobarton, max. 5h. 30m. P.M., min. 8h. 30m. 

 A.M.; at Toronto, principal max. 6 P.M., principal min. 2 A.M., 

 secondary max. 8 A.M., secondary min. 10 A.M. See Sabine, To- 

 ronto, vol. i., p. Ixi., Ixii., and Hobarton, vol. i., p. Ixviii. 



t Sabine, Report on the Isoclinal and Isodynamic Lines in the British 

 Islands, 1839, p. 61-63. 



