358 Mr. Christie on the magnetism oj 
plane. Besides it was manifest that if this were the cause, 
any other impulse would have a similar effect. I therefore 
made the needle revolve first in one direction and then in 
that opposite, by means of a small bar magnet, and invariably 
found that it settled at the same point, in whichever direction 
the impulse was first given, and the results obtained by the 
rotation of the plate were in these cases of the same nature as 
before. It was also evident, that if the deviations I have 
mentioned arose from this circumstance, the needle being 
agitated after any particular point of the plate was brought 
to the limb of the instrument, it ought to settle in the same 
direction, whether that point were brought into this position 
by- revolving from east to west or from west to east; but this, 
except in the cases I have mentioned, where the rotation pro- 
duced no deviation, was not found to take place. In order 
wholly to obviate this objection, in all my future experi- 
ments, after any point had been brought to the limb of the 
instrument, I agitated the needle, and let it settle before I 
noted the deviation. 
T>escription of particular experiments. 
As I had found in my first experiments that I could obtain 
the nature of the deviation caused by the rotation by noting 
the greatest and least deviations when the plate was made 
to revolve in contrary directions, but that the quantity of 
that deviation could not by this means be determined with 
any degree of precision, I resolved to make my future ob- 
servations differently. The method I adopted, when the 
change in the deviation from one point of the plate to another 
was considerable, was this : the plate being placed in any 
