218 



Prof. Hansleen on the Decrease of the 



cal, in order the better to rotate the plates separately on their 

 axes, for the purpose stated in the first page of this letter. 

 The motion of rotation is prevented on ship-board by a steady 

 pin, and proper printed directions are given with the plates,- 

 to insure their being attended to. I remain, Dear Sir, 



Yours very truly, 

 Peter Barlow. 



Art. III. — Observations on the Decrease of the Magnetic In- 

 tensity of the Earth. By Christopher Hansteen, Pro- 

 fessor of Astronomy in the University of Norway. Com- 

 municated by the Author in a Letter to the Editor. 



The horizontal part of the magnetic intensity of the earth 

 here in Christiania, has been almost quite invariable for these 

 six years since 1819- The observation of 300 horizontal vi- 

 brations of my invariable magnetic cylinder, made every day 

 at 10g a. m. and 5 — 7 p. m. in the months of maximum 

 (January) and minimum (June,) has given the following re- 

 sults. The observations were made in three different places 

 of my house, A, B, and C, but all very carefully reduced 

 toC. 



