diurnal variation of the magnetic needle , &c. 353 
daily variation at places in north latitude, to which the above 
positions of the compass and sphere correspond. If the 
instrument was turned 180° in azimuth, so that the elevated 
pole was now to the south of the needle, the effects were 
reversed : that is, when the place of heat was on the meridian 
above the horizon, the deviation of the north end of the needle 
was towards the east ; and it was towards the west, when the 
place of heat was on the meridian below the horizon. So that, 
the deviation of the end of the needle of the same name as the 
latitude was always towards the west, when the place of heat 
was on the meridian above the horizon, and towards the east, 
when it was on the meridian below the horizon. I am not 
aware of any observations having been made on the variation 
of the needle in a high southern latitude, but consider that 
the agreement of the theoretical results with such observations 
would be almost decisive of the correctness of the theory. 
Besides the method which I have described, I adopted others 
for heating the equatorial part of the sphere ; one of these, by 
which the results were greatly increased, was this : having 
covered a copper wire with very loosely spun string, I wetted 
it throughout with spirits of wine, and placed it round the 
sphere, somewhat below the equator, so that, when the spirit 
was inflamed, the equatorial part of the sphere was the hottest. 
The string being in largest quantity where the two ends of 
the wire were joined, and this joint being downwards during 
the combustion, the corresponding part of the sphere became 
the place of greatest heat. To whatever part of the equator 
this joint was adapted, the effects perfectly corresponded 
with, though they greatly exceeded those which I have 
before stated. 
Z z 
MDCCCXXVII. 
