38 Mr. Christie on the Magnetism of hon [July, 



according to our inference from the last observations, deviated 

 in the direction of its rotation ; but according to the inference 

 drawn from the first series of observations, the end of the needle 

 of the same name as this edge, will, in the new position, deviate 

 in a direction contrary to that of its rotation ; so that the rota- 

 tion of the plate being in the same direction in both positions, 

 the deviations by rotation will be in contrary directions in 

 the two cases : and consequently, between the two positions, 

 the plane of the plate must have passed through one in which the 

 rotation would produce no deviation. If we conceive the plate 

 to come into the position of the tangent-plane by revolving 

 ^bout its diameter in the opposite direction, that is, by the inner 

 edge moving towards the pole of a contrary name to the latitude, 

 the inner edge will become the edge of the contrary name to 

 the end of the needle, which in the first position deviated in the 

 direction of its rotation ; and therefore that end of the needle 

 will still continue to deviate in the same direction ; that is, the 

 direction of the rotation being the same in the two positions, 

 the deviation by rotation will be in the same direction in both 

 cases ; and consequently between the two positions, either there 

 is no position of the plane of the plate in which the rotation will 

 produce no deviation, or there are two, or some even number of 

 such positions. 



" I have not been able to determine in all cases experiment- 

 ally, the situation of the plane in which the deviation due to 

 rotation vanishes, or whether there may be more than one plane 

 in which this takes place ; but all the observations which I have 

 made confirm me in the opinion which I formed on comparing 

 the preceding results, that when the centre of the plate is in the 

 meridian, there is only one plane between the tangent-plane and 

 the plane passing through the centre of the needle in which the 

 deviation due to rotation vanishes, and that that plane is parallel 

 to the equator. 



" Another conclusion which we may draw from these experi- 

 ments compared with those above referred to, is this, that when 

 the centre of the plate is in the meridian, and its plane perpen- 

 dicular both to the meridian and equator^^ then, supposing the 

 plate always to revolve in the same direction, the deviation will 

 always be in one direction, in whatever point of the meridian 

 the centre of the plate may be. 



" As I had already found, that when the centre of the plate 

 was in the secondary to the equator and meridian, and its plane 

 a tangent to the sphere, the rotation caused no deviation of the 

 horizontal needle : it appeared to me that there ought to be no 

 deviation due to rotation when the plane of the plate was in any 

 other plane perpendicular to this secondary. To ascertain how 

 far my views were correct, or otherwise, I adjusted the plate on 

 the anu; the same as in the last experiiueatS; and the iustrumeut 



