416 DISAGREEMENT OF MAGNETIC NEEDLES. 



but, considering its lightness and the few times 

 that it had been used as compared with the others, 

 as well as the fineness of the point of the 

 pivot, and observing, moreover, its constancy in 

 returning to zero under various trials, I at last 

 concluded that it must be right, and adjusted 

 the instrument accordingly. With the face of 

 the needle to the face of the instrument, it swung 

 more freely east and west than when turned 

 north and south ; for in the latter position it was 

 sometimes sluggish, and jerked as if acted on by 

 two powers, whereas in the former the motion 

 was smooth and easy. When it was reversed 

 the discrepancy was still more apparent, and in 

 one instance it did not make nearly the same 

 number of vibrations. For this strange devia- 

 tion I can assign no accidental cause : on this 

 occasion, in particular, there was not a particle of 

 iron or any metallic substance within three hun- 

 dred yards of the tent; for, having remarked on 

 other trials the danger of having so much as a 

 pocket knife near while the observations were 

 in progress, I now, to be still more certain, 

 even removed my chronometers, and took off my 

 brace-buckles. Having got the vertical intensity, 

 and then the dip, which agreed better than 

 might have been expected, I tried Hansteen's 

 No. 3. needle for the horizontal force ; but I 

 cannot easily describe the tediousness of arrang- 



