Magnetic Variations at different Places, 



191) 



Table II. Mean Deviation of Time at each Station from the 

 General Mean for all Stations, and Difference between tbe greatest 

 and least Values of Deviation at each Station. 



At Cape Horn and Melbourne the times were given only to the 

 nearest 5 minutes ; at the other stations they were given to the 

 nearest minute. This may explain the greater differences for Cape 

 Horn and Melbourne in the last column of the table. 



The question that now arises is, What is the explanation of the 

 small differences between the values of deviation at different places ? 

 As already suggested, they must be due either to real difference in 

 the time of the magnetic impulse, depending on geographical posi- 

 tion, or to the existence at individual stations of some systematic 

 error. 



In regard to the possible variation of the values of deviation with 

 geographical position, there may be variation with latitude or with 

 longitude. It 'will be observed that in Table II the stations are 

 already arranged in order of longitude, and the run of the numbers 

 would appear to indicate a tendency to negative values at westerly 

 stations, and to positive values, on the whole, at easterly stations, 

 but yet the differences are such (remembering how easily some of 

 these differences might arise, with existing apparatus) as would 

 hardly warrant the assumption that such variation really exists; 

 neither does a change with longitude seem to be one for which there 

 would appear to be any ready physical explanation. The variation 

 in the values of deviation with latitude is betteu seen by arranging 

 the numbers of Table II in order of latitude, and, considering the 

 divergence of the magnetic equator from the astronomical equator, it 

 seems better to group them according to the values of magnetic dip 

 at the various stations, which has been done as follows, adding also a 



