diurnal variation of the magnetic needle^ ^c. 329 
approximation, however, to the nature of these deviations, 
may be obtained by making two magnetic poles revolve, rela¬ 
tively to a needle, in the same manner as I have supposed 
those near the equator. 
For this purpose I made use of an instrument somewhat 
resembling an altitude and azimuth circle. To the vertical 
graduated limb an axis, in the direction of a diameter, is 
firmly attached. Perpendicular to this axis is a graduated 
circle carrying two arms, to the ends of which slender bar 
magnets 6 inches in length are attached perpendicularly to 
the plane of the circle, and so that their upper poles, which 
are of contrary names, are in the plane of the circle. These 
arms are moveable, and may be fixed so that the poles shall 
subtend any required angle at the centre of the circle, which, 
with the magnets, may be made to revolve round the axis. It 
is evident, that if the axis be inclined to the horizon at an 
angle equal to the latitude of the place of observation, and so 
that the azimuth of its lower extremity from south of the 
magnetic meridian be the same as the variation of the north 
end of the needle at that place, then, a needle being placed 
vertically over the centre of the revolving circle, the magnets 
would revolve, relatively to the needle so placed, as the poles 
which I have supposed near the equator, with respect to a 
needle at the place of observation; and the deviations of 
this needle, corresponding to particular positions of the 
magnets, would indicate the nature of the changes due to 
the action of poles, similarly placed, near the equator of the 
earth. 
For the purpose of comparison, I have selected the obser¬ 
vations made by Lieut. Hood in 1821, at Fort Enterprise, 
MDCCCXXVIL 
