314 
Mr. Christie's theory of the 
acting in a peculiar manner on a magnetised needle, or not, is 
not the object of my present enquiry, although the experi¬ 
ments which I shall detail may be of a nature to throw some 
light on this part of the subject. I therefore adopt the term 
pole, as a convenient one, to indicate in a general manner, the 
points towards which the attractive and repulsive forces, 
apparently acting upon the needle, seem to tend; and the 
term axis of polarity, to indicate the line joining two poles, the 
forces towards one of which are attractive, and towards the 
other repulsive, for the same end of the needle. 
My first object was to ascertain that, whatever might be 
the action of the plate, it was tolerably uniform throughout. 
This was done in the first instance by placing the plate with 
its plane in that of the meridian, and the centre of the needle 
in the vertical diameter produced, at the distance *55 inch 
from the upper edge, and then observing the deviations, in 
the manner I have described, when different points of the 
plate had been heated, by applying the lamp to them for two 
minutes, taking care, after a set of observations had been made 
for each heated point, to cool the whole previously to heating 
another. The circumference of the plate was on one face 
divided into eight equal parts, and the points of division num¬ 
bered o, 45, 90, 135, 180, 225, 270, S15, so that this face 
being towards the west and o downwards, 90 was south. 
The observations are contained in the following table. The 
centre of the plate being taken as the centre, the angular 
distances between the centre of the needle and the heated 
point are indicated in the first column: they are measured 
from the vertical towards south, and always in the same 
direction to 360®. The observed deviations of the needle. 
