MAGNETIC OBSERVATIONS 



319 



Inasmuch as Commander Cagni's observations were, with but one exception, made during 

 afternoon hours, his values have also been reduced approximately to mean of day by the same 

 diurnal variation curve (the corrections given are mean values over periods of observation). 

 These results are as follows, the observed quantities being taken from Professor Palazzo' s 

 discussion :* 



Date 



1899 



August 29. 

 August 30. 



igoo 

 July 12.. . . 



July 13.... 



July 14.. . 



July 14.... 



July 24.... 



July 25... 



August 3 . . 



o a 9 



Local 

 mean time 



km h m 

 14 31 - 17 30 



14 46 - 18 02 



17 07 - 17 31 



15 41 - 18 35 

 9 08 - 12 30 



15 37 - 16 01 

 IS lo - 18 35 

 IS 23 - 18 30 

 14 40 - 18 09 



Mean 



observed 



value 



r 



6842 



6S48 



6895 



6892 



6835 



6876 

 6831 

 6881 

 6849 



Correction 

 diurnal 

 variation 



— 41 



— 41 



— 42 



— 41 



— 6 



— 43 



— 39 



— 39 



— 40 



Reduced 

 mean value 



y 



6801 

 6807 



6853 

 6851 

 6829 



6833 

 6792 

 6842 

 6809 



Resulting 



mean 



H 



r 



6805 



6823 



The resulting mean values of 6805 y and 6823 r correspond to mean of day for the epochs 

 1899.66 and 1900.55 respectively. From these the magnetic horizontal intensity at the Italian 

 station for the epoch 1900. 10 is 6814 j-. Referred to the Ziegler Expedition station, this becomes 

 68 14 r — 68 7* = 6746 y. At the latter location the value derived was 6768 ;* for the epoch 1 904. 1 6. 

 Hence the horizontal intensity of the earth's magnetic field has, in the region of Teplitz Bay, 

 in a period of four years increased 22 y (0.00022 C. G. S.). Hence, assuming a linear change 

 during the elapsed interval, the rate of change in magnetic horizontal intensity due to secular 

 variation is 



-\-6y (0.00006 C G. S.) per year. 



This rate of increase confirms that adopted by Professor Palazzo in his discussion above 

 referred to, namely, +9 r- A-t the stations Mossel Bay (^ = 79° 53' N ; A= 16° 04' E) and 

 Cape Thordsen {f = 78° 28' N ; A = 15° 42' E) for the periods 1873.5 to 1899.7 and 1883.2 to 

 1899.7 Dr. Solanderf has deduced the annual rates — 10 ^ and — ^y respectively. 



*Osservazioni scientifiche esequite durante la spedizione Polaredi S. A. R. Luigi Amedeo di Savoia, Duca 

 degli Abruzzi, 1899-1900. Milan, 1903. Pp. 47S-500. (Relazione suUe osservazioni iiiagnetiche fatta dal 

 Frofessora Luig^ Palazzo.) 



tSee reference, page 305. 



