1879.J Observations of the Arctic Expedition, 1875-76. 



33 



r October, 9 p.m., 

 November, 3 a.m., 

 December, midnight, 

 January, 330 a.m., 

 February, 4 a.m., 

 March, 9 p.m. 



A separating value of 30' at a station where the absolute horizontal 

 force is 1*13, and the inclination 84° 50', may be considered sufficient, 

 when compared with those adopted by Sir Edward Sabine, viz., at 

 Point Barrow, 22'"87, with a horizontal force equal 1*88, and dip 

 = 81° 36', at Port Kennedy, 70' with horizontal force equal 0"35, and 

 dip = 88° 27' * 



After rejecting days of assumed disturbance the hours of extreme 

 deflection are : — 



Floeberg Beach.. 



Westerly /December, 1 p.m., Easter l y f December, 1 a.m., 



extreme', i Janua ^' 2 ^ extreme. ^ anuar - T ' 10 P - M "' 



L February, 3 p.m. I February, 2 a.m. 



Discovery Bay. 

 October, 11 A.M., 

 November, noon, 

 Westerly , December, 11 a.m., Easterly 

 extreme. ] January, noon, extreme. 

 February, 11 A.M., 

 1^ March, 2 p.m. 



The mean values for the whole period are . — 



Westerl f ^ P * M ^ an R ensse l aer Harbour 2 a.m. "| j] as ^. er ]_ 



extreme 1 10 A ' M ' anc * 1 P ' M ' Floeberg Beach 1 a.m. ? extreme 



1 11 a.m Discovery Bay . . . Midnight J 



The general effect, therefore, of disturbance is to accelerate the 

 arrival of the magnet at its points of extreme deflection by one hour, 

 in addition to producing a much larger range in the diurnal variation. 

 Thus, with disturbances included, the range is 35'' 6 ; with their re- 

 jection it is only 16'"3 at Discovery Bay. 



The erection of the declinometer houses of both ships on the ice- 

 floe (which rose and fell with the tide), renders the observed motions 

 of the magnet open to the suspicion of disturbances other than those 

 produced by magnetical causes. 



Chiefly for this reason no further analysis has been made of the 

 disturbances at Floeberg Beach. 



In the case of the " Discovery," comparisons were made between 

 the declinometer on the floe and the unifilar magnetometer on shore 

 during times of disturbances, and the magnets moved alike, with two 

 exceptions. On February 2nd, the magnet on shore was much dis- 

 turbed, that on the ice very steady ; at midnight of February 28th, 

 the reverse took place, the declinometer being disturbed in a peculiar 

 manner, whilst the unifilar magnetometer remained nearly steady. 



Analysis of Disturb ances. 



For computing the mean hourly values of the disturbances of the 



* See "Phil. Trans.," 1863, p. 656. 

 VOL. XXIX. D 



