Mr, Christie on the magnetism developed in 
results. The results obtained in the former case appear to 
indicate, that every particle in the copper urges the needle 
from the magnetic meridian with a force varying as 
^di^ance)^’ which law would arise from the magnetism in 
the needle developing the magnetism in the particles of 
copper, so that its intensity would vary as and this 
magnetism again acting on the poles of the needle with a 
force varying as Supposing this to be the case, if z 
is the distance of a lamina of copper from the plane of the 
needle, s the arc of a circle in this lamina at the distance r 
from the axis of rotation, R the radius of the copper cylinder, 
t its thickness, c the distance of its upper surface from the 
needle, and a the distance of the pole of the needle from its 
centre : then the whole force with which the cylinder urges 
the needle will be proportional to 
Although this may be integrable, the integral would be in 
so complicated a form, that it would be very ill suited for 
comparison with the results obtained from observation ; but 
if we consider only the annulus of the copper immediately 
under the pole of the needle, which will be the most efficient 
part, w^e may readily make this comparison. For calling 6 the 
deviation, we should have sin. 0 = x const, or sin. Q 
= ~ (TTlp) consequently - 
= const. 
