MAGNETIC OBSERVATIONS. ill 



mirable work, which is of a more comprehensive nature 

 than my own. 



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 Acapulco, indicate considerable alterations in the 

 inclination (and the longer the intermediate period the great- 

 er 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'; while Captain Belcher, in 1840, 

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

 scarcely amounted, in the course of sixty-seven years, to 

 O''51.# A very careful observer, Sawelieff, found in North- 

 ern Asia, twenty-two 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 of 

 the parallel of 50 had varied very irregularly.! 



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 near- 

 ly half a century.J 



The solution of the question whether the elevation of the 

 soil does in itself exert a perceptible influence on magnetic 

 dip and intensity, was made the subject of very careful in- 

 vestigation during my mountain journeys in the chain of the 

 Andes, in the Ural, and Altai. I have already observed, in 



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



f Compare Sawelieff, in the Bulletin Physico-Mathematigue de FAcad. 

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

 p. 440. 



J 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, 

 who, indeed, always reversed the poles, but who made his observations 

 with a needle which did not move freely, it follows that there has been 

 an increase in the inclination at the Cape of Good Hope of 3-08 in 

 eighty-nine years ! 



6 Arago, in the Annuaire du Bureau des Long, pour 1 825, p. 285-288. 



