t 



6 



Oil the general distribution of Terrestrial 



The most important comparison is that witli Gauss's maps of the 

 computed values of the declination, dip, and horizontal intensity. 

 They are the only ones in which the three elements have been 

 considered as having a neccessary connection witli each other, 

 and while they may be considerably in error as to absolute quan- 

 tities, the agreement inform with the lines on our mapSj derived 

 purely from observation, is strong evidence of the general cor- 

 rectness of the assumptions upon which they are based. 



The data for the declination were taken by Gauss from Bar- 

 low^s map in the Phil. Trans.^ 1833) of which the mean epoch 

 cannot be later than 1830. The system of lines derived by 

 Gauss on theoretical consideration differs in certain localities 

 materially from that of Barlow. The annexed diagram exhibits 



i 



diffi 



W 



o 



ha3 



several branches which diverge tangentially, according to Gauss 

 such a divergence cannot take place ; but when there is a space 

 within which the declination is less than outside any portion of 

 its limiting line, that line must form a loop, the two branches in- 

 tereecting nearly at right angles, as shown in the looped line of 

 ^^ 45'. It must be remembered that the region in qut^stion is 



8 



.de 



ite as much as that of ' 

 graphically performed 



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