1826.] arising from its Rotation. 109 



tlien also, the deviation of the south end of the needle due to 

 rotation, would be in the direction of the north or loicer edge of 

 the plate, and the deviation of the north end, in the direction in 

 which the south or upper edge revolved ; which are precisely the 

 directions of the deviations of the horizontal needle in this 

 position of the plate. 



" The law which I have shown to obtain in all the experiments 

 on the horizontal needle, viz. that the sides of the equator of 

 the imaginary dipping needle always deviated in directions 

 contrary to those in which the corresponding edges of the plate 

 moved, I had derived previously to having an opportunity of 

 making any experiments with the dipping needle : a comparison 

 of the above results with this law will more fully illustrate its 

 nature, and at the same time show their perfect accordance. 

 In making this comparison, it is necessary to notice that, an 

 increase of the dip of the needle, corresponds to a deviation of 

 the southern edge of its equator towards the south pole, and of 

 the northern edge towards the north pole ; and on the contrary, 

 a diminution of the dip corresponds to a deviation of the southern 

 edge of the equator towards the ?iorth pole, and of the norths 

 em edge towards the south pole. Now, when the equatorial south 

 edge of the plate revolved towards the polar south, and conse- 

 quently the equatorial north edge towards the polar north, the 

 inclination of the needle was diminiihed by the rotation ; that 

 is, the south edge of its equator deviated towards the north pole, 

 and the north edge of its equator towards the south pole ; or 

 the edges of the equator, by the rotation of the plate, deviated in 

 directions contrary to those in ivhich the edges of the plate moved. 

 The same conclusion evidently follows from the observations 

 when the equatorial south edge of the plate revolved towards 

 the polar north, the dip being here increased by the rotation of 

 the plate. 



^^ The next observations which I made, were of the inclinations 

 of the dipping needle, when the plane of the plate was in the 

 plane of the meridian or plane of vibration of the dipping needle. 



** From these observations it appears that, the plane of the 

 plate being in the plane of vibration of the needle, and its 

 centre in the equator, the deviation of the upper or south end 

 of the needle, due to rotation, was in the direction of the rota- 

 tion of the upper or south edge of the plate, and of the north 

 end in that of the north edge ; and we may therefore conclude, 

 that if a needle could be freely suspended by its centre of 

 gravity, and the centre of the plate were in the equator, and 

 its plane in that of the secondary to the meridian and equator, 

 the deviation of the south end, due to rotation, would be in the 

 direction in which the south edge of the plate revolved, and of 

 the north end, in that in which the north edge revolved ; which, 

 again, are precisely the directions in which we have seen, that 



