706 



TERRESTRIAL MAGNETISM. 



If the angle ^ is small, then 



2 X \ x^ 

 ^ r 3 • r^ 





and? = ^-i-.^' 

 2 4r 12 r3 





Sothatsin^ = i (x- 

 2 4r V 



11 

 32 • 



11 



hpnce tan i -r- "^ 



x^ 

 r2 



^^ 32- 



^,3 

 ,,2 





11 0^3 

 The correction — ^ • -:? must be applied to the observed read-^ 



ing of the scale. 



The needle being at rest, the coil was turned through 180°, 

 causing a deflection of galvanometer, at the instant when the 

 galvanometer needle was swinging back through its point of rest, 

 the coil was turned back again, thus increasing the swing, and at 

 each instant when the needle is passing its point of rest^ the coil 

 is turned through 180°, each time increasing the swing of the 

 needle until the increase in the swing becomes very small. 



Taking the mean of all these successive corrected deflections 

 of the needle on both sides for the horizontal and vertical posi- 

 tions of the axis, their ratio will give the tangent of the dip. 



Horizontal Intensity, 



Observations of horizontal intensity were made in the usual 

 way, by measuring the moment of inertia of a magnet, its time of 

 vibration, and the amount of deflection which it produces in a 

 small needle placed at given distances from it. 



Observations to determine the values of the magnetic elements 

 were also made at various points of the coast with the several in- 

 struments, and the results are contained in the following table : — 



Place. 



Time. 



Lat. 



Long. 



Declina- 

 tion. 



Inclina- 

 tion. 



Hor. 



Force. 





20 Apr. 1870 



76 23 



19 36 



45 28 



/ 







7 „ „ 



76 18 



19 41 



45 30 









14 Aug. 1869 



75 30 -.5 



17 40 



45 24 







Cape Bergen 



— Bremen - - - 



28 July 1870 



75 26 



18 1 



— 



79 50-1 



0-9608 



80 May „ 



74 58-4 



19 57 



45 48-2 



79 57 



0-9830 



Phil. Broke 



— 



74 55-8 



17 40 



45 25-5 



78 50-6 



1-1325 



Klein Pendulum - 



29 Aug. 1869 



74 87-6 



18 29 



43 3-6 



80 10-4 



1-0614 



Sabine Island 



— 



74 82-3 



18 49 



45 6-7 



79 42-4 



1-0489 



Cape Broer Ruys 



4 Aug. 1870 



73 27-7 



20 43 



43 52-5 



79 54 



0-9832 



Franz Jos. Piord 



12 „ „ 



73 11-6 



25 58 



50 30-2 



79 59*7 



0-9587 



The Inclination observed at Sabine Island, in East Greenland, 

 in 1823, by Sabine was 80° IT, and the Total Intensity observed 

 by Sabine 11 '54 in absolute British units. 



