Mechanical Science. 607 



The intensity of that part of the magnetic force which acts in a 

 horizontal direction is therefore, at St. Petersburgh, 



By needle A s= ( 3 - 09 go ~ 0.933) the same at 

 (3.208 I Berlin being 



By needle B = < 4 - 6161 )' = 0.931 



(4.7852) 8 



In order to deduce from these results the ratio of the entire force 

 at the two places, the dip at Berlin was taken at 68 39' 30", as de- 

 termined in the same manner, and by the same instrument with 

 which the preceding result for St. Petersburgh was obtained. "With 

 these data, and using the mean of the results deduced from the two 

 needles, the following expression is found for the intensity of the 

 magnetic force at St. Petersburgh ; that of Berlin being considered 

 as unity : 



0.932. secant r (71 12' 25") _ . * 



secant (68 39' 30") " 



11. VARIATION OF THE NEEDLE. 

 (From a Letter of Professor Hansteen to M. Kupjfer.) 



Orenburg, Jan. 1, 1830. 



The magnetical observations in the eastern parts of Siberia prove 

 that there is a considerable western variation in places eastward of 

 the line of no variation which passes near Irkutsk. Professor Han- 

 steen has likewise discovered the line of no variation which had 

 already been established by the observations of M. Schubert. The 

 observations of Captain Wrangel had led M. Kupffer to believe that 

 the variations on both sides of this line of no variation had equal 

 signs ; but the very accurate observations of M. Hansteen now 

 prove, that they change sign from one side to the other of that line ; 

 and that, consequently, this line of no variation which traverses 

 Siberia has the same property as those which pass near Kasan, and 

 through the United States of America f. 



12. ON THE FIGURE OF THE MAGNETIC EQUATOR. 



The following are deductions from observations made during the 

 voyages in the Coquille, by L. J. Duperrey, commander of the ex- 

 pedition. 



It had been previously known that the magnetic equator, or that 

 line surrounding the globe where the needle arranges itself in a 

 horizontal position, was not a regular great circle of the sphere, but 

 an irregular line cutting the equator in two points. From the obser- 

 vations made by M. Duperrey, combined with those of Captain 

 Sabine and others, its true figure may now be considered as ascer- 

 tained with a considerable degree of accuracy. M. Duperrey crossed 

 the magnetic equator six times ; but it was not so much from obser- 

 vations made on that line, as from those made about thirty degrees 

 * Petersburgh Transactions, 1830. f Idem, 1831, 



