TEKRESTRIAL MAGNETISM. 117 



10 P.M. to 2 A.M. a secondary maximum occurs at both 



stations. 



The four turning hours of the inclination at Toronto 

 are almost exactly reproduced at Hobarton, only the sig- 

 nification is altered. This complex action is very deserving 

 of attention ; as is also the comparison of the two stations 

 in respect to the sequence of the turning hours of the varia- 

 tions of the inclination and of the total force. ( 136 ) 



The periods of the inclination at the Cape of Good Hope 

 do not agree either with Hobarton, which is in the same 

 hemisphere, or with any of the northern stations which have 

 been referred to. The minimum of inclination even takes 

 place at an hour when the inclination at Hobarton has 

 almost reached its maximum. 



The determination of the secular change of the inclination 

 requires observations of equal and satisfactory accuracy, 

 repeated so as to include long intervals of time. We do 

 not, for example, find that we can go back to Cook's 

 voyages with the desired degree of certainty ; for although 

 in his third voyage the poles of the dipping-needle were 

 always reversed, yet differences from forty to fifty-four 

 minutes occur between that great navigator's observations 

 in the Pacific and those of Bayley, attributable probably to 

 the then very imperfect construction of the needles, and 

 especially to their want of free movement. For London, 

 we are reluctant to go back beyond Sabine's observations in 

 August 1821, which, compared with the excellent deter- 

 mination by James Ross, Sabine, Johnson, and Fox, in 

 1838, gave for that interval an annual rate of decrease of 

 2'*73 ; in near accordance with which, Lloyd, with equally 



