1826.] during the Act of Rot at ion > 447 



had no motion. I tried also a variety of other positions, but I 

 could obtain no such results as to lead to a concise expression 

 of the effect, and for this reason I shall not trouble you with the 

 detail of them. 



" It at length occurred to me, that the reason of my failure 

 arose from the compound influence under which the needle was 

 placed, viz. that of the iron ball and of the earth ; I therefore 

 now neutralized it from the effect of both, by means of magnets 

 properly disposed, adjusting it always before the rotation to a 

 direction tangential to the ball, so that whatever effect was pro- 

 duced at each point, might at least become decided as to its 

 direction. I now immediately arrived at that kind of general 

 law I had been in search of; for I found when things were thus 

 arranged, that whatever might be the direction of the axis of 

 rotation, if the motion of the ball were made towards the needle, 

 the north end of the latter was attracted ; and if from the needle, 

 the north end was repelled by the iron, the points immediately 

 in the axis (when of course the motion of the shell was parallel 

 to the needle) being neutral, or those at which the change of 

 direction took place ; in other words, if the motion of the shell 

 continue the same, and the compass be successively placed all 

 round the ball, in that semicircle (from one axis to the other) in 

 which the motion is towards the needle, the north end ap- 

 proaches the ball, and in the other semicircle it recedes, or the 

 south end approaches; the points of non-action being in the two 

 extremities of the axis, and those of maximum effect in two oppo- 

 site points at right angles to the axis ; in which two latter, the 

 needle, when properly neutraUzed, points directly to the centre 

 of the ball. 



. "This will be perhaps better understood by reference to fig. 3, 

 where S is the shell, a b its axis, and u s, n s, &c. the needle in 

 its various positions prior to the motion, and u' s\ n' /, &.c. its 

 direction as resulting from the motion; the rotation of the shell 

 being from c towards (L . of course with the rotation reversed, 

 the effect will be reversed also." 



The author here observes, addressing Mr. Herschel, " Now 

 this effect you will, I think, fmd to be perfectly consistent with 

 the view you have taken of the subject, in your letter of Jan. 1!^, 

 where you say in reference to your former query, and to the 

 views I then entertained, ' I should rather have expected a 

 diminuUon of the magnetic polarity, commensurate to the 

 rapidity of rotation and a change in the direction of the mag- 

 netic axis of the globe, from parallelism to that of the earth, to 

 a position somewhere intermediate between that and the axis of 

 rotation, but approaching nearer the latter as the velocity 

 increased. Sec' 



" The fact is, that the needle in my experiments being under 

 no influence, prior to the rotation, from either the iron or tlie earth, 



