diurnal variation of the magnetic needle^ &c. 319 
If these deviations are to be considered as due to polarity 
in the plate, the situations of the poles may be deduced 
from them, pretty nearly, without entering into any calcula¬ 
tions ; for which purpose indeed the observations could 
scarcely be made with sufficient precision. For example: the 
needle being below the centre, and the heated point opposite 
to it, it is evident, from the character of the deviations, that, 
looking from the compass on to the plate, either there must 
-be a south pole to the right of the needle, or a north pole to its 
left; or a south pole to the right and a north pole to the left. 
As however the direction of the deviation is changed by 
making the heated point describe a quadrant, on either side of 
the compass, both these poles must exist, the south pole being 
within the lower quadrant on the right, and the north pole 
within that on the left. The positions of these poles are 
more precisely fixed by some of the other observations. 
When the compass was opposite to the line of junction of the 
copper and bismuth, above the centre of the plate, and to the 
north of its surface, the angular distance of the place of heat 
from the centre of the needle being 180°, the deviation was 
23° 30' east; when this distance became 270®, the place of 
heat being 90° to the east, or to the left of the compass, the 
deviation became 164° 30' east; and as the place of heat ap¬ 
proached the needle, the north end passed through south, the 
deviation becoming N 47“ 3o' W, when the place of heat was 
opposite to the compass. This effect is precisely that which 
would have taken place had a strong 7 iorth pole existed in 
the north, or marked surface of the plate, in the radius form¬ 
ing an angle of about 64° with the axis of heat, to the left or 
east, supposing the place of heat downwards ; this north pole. 
