359 
iron arising from its rotation. 
required position, I made it revolve once, for example, the 
upper edge from east to west, without noting the deviations, 
bringing the point marked o° to coincide with the line indi- 
cating the position for observation ; from hence I continued 
the revolution of the plate until the point marked 30® 
coincided with the same line, and, after slightly agitating the 
needle, noted the deviation ; and in the same manner were 
the points 60“, 90°, 120®, 150®, 180®, 210°, 240®, 270°, 300®, 
330®, 360° or o® brought successively to coincide, and the 
deviations noted. I now made the plate revolve once from 
west to east, without noting the deviations, bringing 0° or 
360° to coincide with the same line, and then brought in 
succession 330®, 300°, 270°, 240°, 210°, 180°, 150°, 120°, 90®, 
60®, 30°, o® to coincide, noting the deviations as before. The 
sum of the first set divided by 12, I considered as the mean 
deviation, when the plate revolved from east to west; and 
the sum of the others divided by 12, as the mean deviation, 
when the plate revolved from west to east : their difference 
was the mean effect of the rotation in contrary directions. 
This I call the Deviation due to Rotatio?i ; and to distinguish 
it from the deviation caused simply by the position of the 
iron, I call this last the Absolute Deviation. When the change 
in the deviation from one point of the plate to another was 
not so considerable, I made the observations only for the 
points o®, 90®, 180°, 270® on the plate. 
I now proceed to the detail of the experiments, and the 
conclusions I draw from them. In those which I shall first 
describe, the centre of the plate was always in the magnetic 
meridian ; its plane was perpendicular to the meridian, and a 
tangent to the sphere, whose centre was the centre of the 
