TERRESTRIAL MAGNETISM 319 



calculation were not much greater than the errors of observa- 

 tion. In Taylor's Scientific Memoirs, vol. ii., will be found maps 

 giving the lines of equal dip, equal declination, and equal total 

 intensity according to Gauss's formula. Considering that the 

 observations could not all be correctly reduced to the same epoch, 

 these lines show a remarkable agreement with the observed lines 

 and justify the preliminary assumption that the earth's field is, at 

 any rate, mostly due to internal magnetism. Subsequent recalcula- 

 tion of the constants * has tended to confirm this assumption. 



Gauss himself suggested that the disturbances of the magnetic 

 elements might be treated in a similar way, and that it might be 

 shown whether they arose from internal or external changes in 

 magnetism. Schuster f undertook such an investigation on the 

 diurnal variation for the year 1870. His work led to the con- 

 clusion that the variation was chiefly " due to causes outside the 

 earth's surface, and probably to electric currents in our atmosphere," 

 and that "currents are induced in the earth by the diurnal varia- 

 tion which produce a sensible effect, chiefly in reducing the 

 amplitude of the vertical components and increasing the amplitude 

 of the horizontal components." 



* Km-y. Itrit., 10th ed., xxx. p. -462 ; llth ed., xvii. p. 881, 

 t Phil. Trans., A, vol. clxxx. (1889), p. 467, 



