1826.] arising from its Rotation, T^ll' 



point our theory is unquestionably at variance with the actual 

 circumstances of the case ; for we have supposed that no partial 

 magnetism exists in the iron, or that every part of it taken 

 separately would equally affect the needle. It is, I believe, 

 scarcely possible to procure iron that shall possess this uni- 

 formity of action, and it is evident that this was not the case 

 with the plate of iron which I made use of. This species of 

 polarity in iron is of so variable a nature, since by an acci- 

 dental blow it will be transferred from one point to another, 

 that it does not appear possible in any manner to submit its 

 effects to calculation. It was to prevent these effects embar- 

 rassing the results, that I took the mean of twelve observations 

 for each position of the plate ; still it is possible that some of 

 the differences between the observations and the results of the 

 theory may have arisen from this cause. 



'* As the results of the hypothesis which I have advanced do 

 not precisely agree with the observations, it will be proper to 

 enquire whether we shall obtain a more perfect agreement by 

 means of the hypothesis commonly assumed, in order to account 

 for the effects produced on the needle by a mass of soft iron, 

 viz. that the upper part of every mass of iron acts as a north 

 pole and the lower part as a south pole. Let us then suppose 

 such poles to exist in the iron plate, in the diameter in the 

 direction of the dip, and that the rotation causes the line joining 

 them to describe in the iron an angle \Jr from this diameter." 



The agreement between the observations and the calculated 

 results from this theory, Mr. Christie here finds, would not be 

 greater than in the former case. 



" In the explanation of the phaenomena which take place on 

 presenting the different ends of a mass of iron to the poles of a 

 magnetic needle, in addition to the hypothesis, that the upper 

 part becomes a north, and the lower a south pole, by position, 

 it is necessary to suppose also, that in every change of position 

 of the iron there is a corresponding and immediate change of 

 its pole ; that is, the upper end becoming the lower, it also 

 immediately becomes a south pole. Now it appears to me, 

 that if we attempt to explain, on this hypothesis, the phaeno- 

 mena arising from the rotation of the iron, we shall find that 

 there are circumstances which are wholly incompatible with it. 

 If on turning a mass of iron end for end, the poles are imme- 

 diately transferred from one end to the other, how can we sup- 

 pose that the revolution of the iron will cause these poles to 

 move forwards, so that the line joining them shall describe an 

 angle from the line of the dip ? or even granting that during the 

 revolution of the iron they may be carried forward, they must, 

 as soon as the iron ceases to revolve, resume their original po- 

 sition in the line of the dip, if they are so immediately trans- 

 ferred from one end of the iron to the other, as it is necessary 



