375 
iron arising from its rotation. 
Although the deviations due to rotation are here 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 wbuld 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 re- 
volved 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 incorrect- 
ness 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 con- 
sidered 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. All 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 conclusions from theory. 
