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gical observations are to be made, which cannot fail to elucidate some 

 of the most important questions in this useful science. 



The magnetic changes to be investigated are of three kinds : first, 

 those which, depending upon a cause not yet satisfactorily explained, 

 take place slowly but regularly, causing a general displacement of 

 the lines of equal variation and dip ; secondly, those which, depend- 

 ing upon the position of the sun, run through their period of change 

 in a year or day, producing different values in the magnetic elements, 

 according to the season or to the hour of the day; and thirdly, the 

 small disturbances which appear to be constantly taking place, and 

 which require for their measurement continued observation with the 

 most accurate instruments. 



The striking fact was proved in 1818, by the observations of Arago 

 at Paris, and of M. Kupffer at Kasan, that the large changes which 

 take place in the position of the horizontal needle during the day, are 

 simultaneous at these places, so distant from each other ; and a con- 

 firmation of the fact as applying to even more distant stations, resulted 

 from the system of observations established by Humboldt and others 

 in 1830, and extended, through the influence of the Imperial Academy 

 of Sciences of St. Petersburg, to the most remote parts of the Russian 

 empire, and even to Pekin. In 1834, the celebrated German philoso- 

 pher Gauss, invented an instrument for measuring the variation of the 

 needle and its changes, which introduced into these determinations an 

 accuracy similar to that attainable in astronomical measurements. 

 This instrument was soon furnished to different observatories, and 

 a- concerted system of observations of the minute changes of varia- 

 tion was introduced, which is now going on at no less than twenty- 

 three places in Europe, the smaller and larger states having vied 

 with each other in providing the means of executing them. The 

 stations include Altona, Augsburg, Berlin, Bonn, Brunswick, Breda, 

 Breslau, Cassel, Copenhagen, Cracow, Dublin, Freyberg, Gottingen, 

 Greenwich, Halle, Kasan, Leipsic, Marburg, Milan, Munich, Naples, 

 St. Petersburg, and Upsala. 



The results already obtained and published by the German Mag- 

 netic Association, have proved satisfactorily that the minute changes 

 in the direction of the needle, as well as the larger ones, are simulta- 

 neous at the different stations, varying however in amount, and the 

 variation appearing to decrease in passing southward ; but the influ- 

 ence of the position of the place, whether depending upon geographical 

 or magnetic position, not having yet been fully determined, and being 



