RESIDENCE AT BEKLIN TO THE REVOLUTION OF JULY. 147 



tunity of arranging for the recommencement of a series of ob- 

 servations, similar to those taken in 1800 and 1807, upon the 

 hourly variation in the inclination of the needle and the periods 

 of extraordinary disturbance in the earth's magnetism. He 

 threw himself into the scheme with renewed vigour, and drew 

 out a comprehensive plan which had for its object the employ- 

 ment of a larger number of observers and the use of superior 

 instruments. For this purpose, Humboldt caused to be con- 

 structed during the autumn a magnetic hut, since become so 

 noted ; it was built entirely without iron, and was erected in 

 the garden of his friend Mendelssohn-Bartholdy, father of the 

 distinguished musician, at his house in Leipziger Strasse. He 

 devoted himself with the most assiduous zeal to these 

 fatiguing observations, which were carried on simultaneously 

 with those of Reich, who conducted his investigations at 

 Freiberg in a mine at a depth of 216 feet; subsequently simi- 

 lar observations were undertaken at Kasan, Nikolaiev and St. 

 Petersburg. In writing to Schumacher on March 13, 1829, 

 he states : ' My hourly observations upon the declination of 

 the needle have been carried on very regularly since the 1st 

 of January. I have observed from hour to hour for whole 

 nights together, and I am anxiously looking for some obser- 

 vations of the aurora borealis at Copenhagen. From the 24th 

 to the 26th of March my hourly observations will be made 

 simultaneously with those at Freiberg, where Grambey's instru- 

 ment is placed in one of the mines.' On March 19 he sent 

 to Reich some observations he had made in conjunction with 

 Paul Bartholdy, Humboldt conducting them through the night 

 and his friend during the day. In a letter written on the 

 26th he says : ' The day before yesterday I again observed the 

 declination of the magnetic needle every hour, simultaneously 

 with th^e Freiberg observations, for thirty-three hours in suc- 

 cession.' Assistance in the work became at length absolutely 

 necessary, as during seven periods of the year Humboldt ar- 

 ranged that observations should be made at least every hour 

 for forty-four hours consecutively. He availed himself of the 

 assistance of Dirichlet, Dove, Encke, Magnus, Pqggendorff, and 

 others, and committed to them the conduct of the observations 

 during his absence in Siberia. Upon the sale of the property 



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