1826.] amitiigj^MiukdUHon. lU 



plane, the equator, was measured on a circle of 9 inches radius 

 parallel to the meridian, passing through the centre of the 

 needle, and at the distance 1*45 inch from it, so that the 

 centre of the needle was always at this distance from the edge 

 of the plate, east or west. As the needle was only two inches 

 in length, and the rim of the compass divided into degrees, the 

 direction of the needle could not be observed nearer than to 5\ 

 and indeed scarcely to that degree of accuracy. The mode 

 which I was under the necessity of adopting in adjusting the 

 compass to the several positions did not admit of extreme accu- 

 racy, so that these positions may be considered as liable to 

 errors amounting to 1°, or perhaps rather more, in angular dis- 

 tance from the equator ; but as my principal object was the 

 comparison of the deviation due to the slow and rapid rotation 

 of the plate, when its centre was in precisely the same positioa 

 with respect to that of the needle, this was not very material ; 

 it will however account for any disagreements that may be 

 noticed in the absolute deviations in corresponding positions, 

 as the greatest accuracy of adjustment would be requisite for 

 their perfect agreement, when the plate is so near to the pole* 

 of the needle. 



" Having ascertained, by the observations when the plate 

 was to the west of the needle, that the rapid rotation produced 

 no permanent change in the iron beyond that arising from the 

 slow rotation, the deviations when any particular points of the 

 plate were opposite to the needle being, as near as could be 

 expected, the same after the rapid rotation as they were after 

 the slow rotation in the first instance, the errors being some- 

 times in excess, sometimes in defect, I did not repeat the obser- 

 vations on the effects of the slow rotation after the rapid, when 

 the plate was to the east of the needle." 



From the inspection of the tables containing these observations, 

 " it appears that the forces which are exerted on the needle during 

 the rapid rotation of the plate, are always in the same direction as 

 the forces which are derived from the slowest rotation, and which, 

 continue to act after the rotation has ceased; but that the 

 former forces are greater than the latter, there being only 

 one instance of the contrary, and that in a position where the 

 effects are so small, that a trifling error of observation would 

 account for the difference. Taking a mean of all the obser- 

 vations, these forces appear to be in the ratio of 19 to 13, or 

 very nearly 3 to 2. It is evident then that the polarising of the 

 iron in the same direction will account for the pheenomena in 

 both cases, but that the intensity of the polarity during the 

 rapid rotation is greater than of that which appears to be per* 

 raanent after the rotation, whether slow or rapid, has ceased ; 



i2 



