GAUSS AND AA'EBER ON TERRESTRIAL, MAGNETISM. 21 



of the activity of these forces from the remark of M. Arago, 

 that frequently on the same days, when he had observed at 

 Paris violent disturbances of the regular movement of the mag- 

 netic needle, northern lights, not visible above the horizon of 

 Paris, had been seen at distant places. 



The irregularities in the phsenomena of ten'estrial magnetism, 

 the frequent occurrence of w^hich had also been observed, espe- 

 cially by Humboldt in his numerous observations of the diurnal 

 and horary oscillations of the magnetic needle, thus obtained a 

 peculiar interest. Though the facts which had been remarked, 

 neither proved that all irregular oscillations of the needle are 

 contemporaneous with the northern lights, nor precluded the 

 possibility, that many, perhaps most of them, have merely local 

 causes, yet it was scarcely possible to mistake the evidence of 

 the not unfrequent action, over a wide extent, of great natural 

 forces, which, if they could not yet be investigated in their 

 sources, offered at least a worthy object of natural inquiry, in re- 

 spect to the relations of their activity and extent. 



Superficial and merely accidental recognitions of such relations 

 can bring us no nearer to this goal : in order to attain it, many 

 such phasnomena must be contemporaneously followed up in 

 accurate detail at numerous stations, and their time and mag- 

 nitude closely ascertained and measured. For this purpose, 

 however, previously concerted plans are essentially necessary 

 among those observers who have suitable means at their dis- 

 posal. 



The celebrated philosopher to whom we are indebted for so 

 many additions to our knowledge of terrestrial magnetism was 

 also here the first to lead the way. M. von Humboldt caused to 

 be erected in Berlin, towards the end of the year 1828, a small 

 house, free from iron, — placed in it a variation compass construct- 

 ed by Gambey, — and concerted with possessors of similar in- 

 struments at various places, some of which were very distant, 

 regular obsen'ations of the magnetic variation on fixed days. 

 Eight terms in the year were agreed upon, each of forty-four 

 hours, dui'ing which the variation was to be noted from hour 

 to hour ; at some places observations were made within still nar- 

 rower limits of time, viz. at every half-hour, or every twenty 

 minutes. The details will be found in the nineteenth volume 

 of Poggendorft*'s Annalen der Physik, p. 361 ; and in the same 

 journal are also the observations which, according to this agree- 



