335 
diurnal variation of the magnetic needle , &c. 
be represented by the rotation of two poles, as I have described, 
I placed the small compass I had before made use of vertically 
over the centre of the circle in the plane of which the upper 
poles ©f-the magnets were to revolve, and adjusted the arms 
carrying the magnets, so that the upper poles were at the 
distance 10-25 inches from that centre. In the first instance, the 
distance of the centre of the needle above that of the circle 
described by the poles was 12-25 inches ; and after observa- 
tions had been made with the compass at this distance, it was 
lowered until the distance was 10-25 inches, and again until 
the distance was 8-25 : the first of these distances corresponds 
to the poles being developed interior to the surface of the 
earth ; the second on the surface ; and the third exterior to 
it. Observations were at first made with the arms carrying 
the magnets so adjusted, that the angle between the poles or 
their difference of longitude was 130°; and the observations 
were repeated when this angle had been altered to 170°. 
For the observations corresponding to those at Fort Enter- 
prise, latitude 64° 28' N. variation 36° 24/ E, the axis about 
which the poles revolved was inclined to the horizon at an 
angle of 64° 28', and the azimuth of its lower extremity 
from magnetic south was made 36° 24' E : so that, the north 
end of the needle pointed N 36° 24' E from the plane of the 
circle representing the true meridian. Adjustments similar 
to these, and corresponding to the latitudes of the places and 
the variation, were made for the observations to be compared 
with those at London, variation 19 0 W, and variation 24°4o' W, 
and at Port Bowen, latitude 73 0 14' N, variation 124 0 W. 
The observations which I made of the deviation of the 
needle, under these different circumstances, at every change 
