317 
diurnal variation of the magnetic needle^ 
formity of action in the plate, I invariably observed the 
deviations corresponding to the heating of different points. 
In some positions of the'compass, observations were made 
when o, 45, 90, 135, 180, 225, 270, 315 had each been the 
heated point, and likewise when the plate had been reversed; 
but in no instance did the results differ in character, or even 
the corresponding deviations vary much in their extent, to 
whatever point heat had been applied. 
To determine the laws which govern the magnetic pheno¬ 
mena resulting from the application of heat to a point in such 
a metallic combination as I made use of, I first adjusted the 
plane of the plate at right angles to the plane of the meridian, 
and placed the compass with its centre in the plane of the 
meridian passing through the centre of the plate, with its 
centre 6 inches above that centre, or opposite to the outer 
edge of the copper ring, arjd 1-3 inch to the north of the 
plane of the plate, that is, as near to it as the compass-box 
would, without touching, admit. The lamp having been applied 
for 5 minutes to the outside of the copper ring at the point 
o, which was the lowest, the deviations of the needle corres¬ 
ponding to the different angular positions of the heated point 
were observed, as in the preceding instance. Corresponding 
observations were made when the compass was in the posi¬ 
tion diametrically opposite, or below the centre of the plate ; 
and also when in the same horizontal plane with that centrCj 
to the east or to the west of it, opposite the outer edge of the 
copper, at the same distance, 1*3 inch north from its surface: 
and similar observations to these were likewise made with 
the centre of the needle, in the corresponding positions, to 
the south of the plate. Observations also, of precisely the 
same nature as these, were made with the centre of the 
