MAGNETIC OBSERVATIONS. 113 



Although Sir Edward Belcher's observations for the year 

 1838, when compared with those I made in 1803 (see p. 73), 

 along the Western Coast of America, between Lima, Guaya- 

 quil, and A capulco, indicate considerable alterations in the 

 inclination (and the longer the intermediate period the 

 greater is the value of the results), the secular variation of 

 the dip at other points of the Pacific has been found to be 

 strikingly slow. At Otaheite, Bayley found in 1773, 29 43' 

 and Fitzroy in 1835, 30 14', whilst Captain Belcher in 1840, 

 again found 30 17', and hence the mean annual variation 

 scarcely amounted, in the course of 67 years, to O'.Sl. 40 A 

 very careful observer, Sawelieff, found in Northern Asia, 

 22 years after my visit to those regions, in a journey which 

 he made from Casan to the shores of the Caspian Sea, that 

 the inclination to the north and south 01 the parallel of 50 

 had varied very irregularly. 41 



Humboldt. Sawelieff. 



1829. 1851. 



Casan .. 68 26'.7 .. .. 68 30'. 8 



Saratow.. 64 40.9 .. ..64 48.7 



Sarepta . . 62 15 .9 . . . . 62 39 .6 



Astrachan 59 58 .3 .. ..60 27.9 



For the Cape of Good Hope we now possess an extended 

 series of observations, which if we do not go further back 

 than from Sir James Ross and du Petit Thouars (1840) to 

 Vancouver (1791), may be regarded as of a very satisfactory 

 nature in respect to the variation of the inclination for nearly 

 half a century. 42 



of successive years we see that the dip passes in one of the turning hours 

 that of the maximum from a decrease to an absolute increase, whilst 

 in the turning hour of the minimum, the progressive annual decrease 

 continued the same. Thus, at Greenwich, the magnetic inclination in 

 the maximum hour (9 A.M.) decreased in the years 1844 and 1845, 

 while it increased at the same hour from 1845 to 184P, and continued 

 in the turning hour of the minimum (3 P.M.) to decrease from 1844 to 

 1846 (Airy, Magn. Observ. at Greenwich, 1846, p. 113). 



40 Phil. Transact, for 1841, pt. i, p. 35. 



41 Compare Sawelieff, in the Bulletin Physico-Mathematique de T Acad. 

 Imp. de St. Petersb. t. x, No. 219, with Humboldt, Asie Centr. t. iii, 

 p. 440. 



42 Sabine, Magn. Observ. at the Cape of Good Hope, vol. i, p. Ixv. If 

 we may trust to the observations made by Lacaille for the year 1751, 



VOL. V. I 



