448 Mr. Barlow on the Magnetic Effect induced in Iron [JuNfi, 



the direction which it takes up in consequence of the motion, 

 enables us to discover the precise direction of the new forces 

 thus impressed upon the shell, and it will be seen immediately 

 to indicate a polarization of the latter in the direction c d; that 

 is, in a direction perpendicular to the axis of motion, and to the 

 plane passing through that axis and the actual poles of the ball. 



" You will of course understand that I do not mean that such 

 a polarization actually takes place ; I mean merely that the 

 cohesive power of the iron is such, as to resist in a certain degree 

 the inductive powers of the earth, whereby the magnetic forces 

 are changed, as you have suggested, from their original direc- 

 tion, parallel to the magnetic axis of the ball, into a position 

 oblique to it, which oblique forces being resolved into two, the 

 one parallel to the original axis, and the other perpendicular to 

 it, and the former being nearly neutralized by the magnets used 

 for the purpose in the first instance, the perpendicular forces will 

 act upon the needle in the same manner as if the ball were 

 really polarized in the direction above alluded to. 



" Having got this view of the subject, I soon found that many 

 of my former results, which appeared to have scarcely any con- 

 formity among themselves, were perfectly consistent with this 

 hypothesis : of these the experiments given above, before the 

 needle was neutralized, may be mentioned. In these I found 

 the point of change to be at about 30° on each side of the axis, 

 so that the arcs in which similar effects were produced were 

 divided into the unequal portions of 60°, 120°, 60°, and 120°, 

 which appeared to be anomalous ; but according to the view 

 now taken of the subject, this is perfectly consistent ; it is pre- 

 cisely what ought to happen according to the law, tan. dip. = 

 2 tan. mag. lat. and which actually takes place on the earth. 

 That is, in passing from the magnetic equator 30° towards the 

 pole, the dipping-needle has actually described a quadrant, as 

 referred to its position at the equator ; and it would describe a 

 quadrant, in an opposite direction in going 30° towards the other 

 pole; so that in passing through 60° the needle is actually 

 inverted ; but if we start from mag. lat. 30° through the pole, we 

 must pass through an arc of 1 20° before the direction of the 

 needle is inverted, and the same in the other half of the meri- 

 dian ; and in like manner by referring the motion of my needle 

 as induced by the rotation of the shell to its original magnetic 

 direction, it is obvious that I ought to have found, as I actually 

 did before I was aware of the cause, a point of change at 30° 

 distance on each side of the meridian passing through the axis ; 

 which meridian, as respects the induced power, is actually the 

 equator of the new magnetic sphere. 



" To render this more obvious, let us refer to fig. 4, in which 

 A B represents the axis of rotation of the shell, the black lines 

 the needle in its natural direction, and the dotted liues the 



