1826.] arising from its Rotation, 39 



so that the axis M Q being in the plane of the meridian and 

 inclined to the horizon at an angle of 19° 30', the centre and plane 

 of the plate were, during the revolution of the limb, always in 

 thie position I required. The distance between the centres of 

 the needle and plate was, as before, 13-2 inches." 



^' Although the deviations due to rotation in these observations 

 are in some cases greater than might perhaps on a first view be 

 expected, if in the position in which I have supposed the plate, 

 its rotation would really produce no deviation, yet the differ- 

 ences are not in any case more than may, I consider, be fairly 

 attributed to errors in the adjustments. That the deviations, 

 when the plate revolved from south to north, had a tendency 

 most generally to be greater than when it revolved in a contrary 

 direction, as is evident by referring to the Table, appears at first 

 sight more unfavourable to my opinion than the magnitude of 

 the difference ; but on further consideration, I think that this 

 will be allowed rather to point out the source of the errors in 

 the results, than the incorrectness of my views, and that these 

 errors arose from the plane of the plate not being in those cases 

 perpendicular to the plane of the secondary to the equator and 

 meridian. The proximity of the edge of the iron to the ends of 

 the needle, varying from 5*16 inches to 4*27 inches at the south 

 end, and from 5-16 inches to 5*92 inches at the north end, I 

 considered to be another source of error ; the inequalities arising 

 from the effects of particular points near the edges of the iron 

 on the ends of the needle being the more sensible when the 

 distances are small. AH my observations were made as near to 

 the centre of the needle as the instrument would admit, in order 

 that the effects of the rotation, since they were in many cases 

 extremely small, might be the more sensible ; and by this means 

 I discovered the nature of the effects produced on the needle by 

 the rotation of the plate; but I am fully convinced, that for the 

 purpose of comparing the results of observation with the conclu- 

 sions from theory, it is always desirable, that the observations 

 should be made when the iron is at such a distance from the 

 centre of the needle, that the effects of particular points near its 

 edges, on the ends of the needle, are nearly insensible. Taking 

 these circumstances into consideration, 1 was quite satisfied 

 from these experiments, that, if the centre of the plate be in the 

 secondary to the equator and meridian, and its plane perpendi- 

 cular to the plane of that circle, the rotation of the plate will 

 produce no effect on the absolute deviations caused by the 

 mass. 



" In order to determine what effects would be produced by the 

 rotation of the plate when its centre was in the secondary to the 

 equator and meridian, and its plane in the plane of this circle, 

 the instrument was adjusted as in fig. 1, the index at g pointing 

 to 70"^ 30'; the limb S ^ N was then placed at right angles to 



