OF TERRESTRIAL MAGNETISM. 
201 
not seem easy to account for the discrepancies which not unfrequently oc- 
curred in the indications afforded by the two needles at the same time, and I 
hesitate to adopt any distinct conclusions from this circumstance without 
further experiments, which I intend to make with horizontal needles on the 
same principle. 
I have endeavoured by various experiments to ascertain whether there is 
any decided and permanent difference in the directive force of the opposite 
poles ; — for example, I have alternately neutralized in an equal degree the 
poles of a needle by means of a sliding axis, and suspended it horizontally by 
unspun silk, but the two ends vibrated in nearly equal times. Other needles 
on the same principle gave corresponding results, or at least the differences 
were so inconsiderable, and were so nearly compensated on an average of 
many observations at different times, that I think it may be safely assumed that 
there is an equality in the yearly mean of the magnetic intensity of the two 
poles separately considered. But on the hypothesis of a central magnetic force, 
ought not the north pole in this latitude to be acted on with much greater 
energy than the south ? for if my experiments may be depended upon, the 
alternative can scarcely be adopted, of supposing that one pole of a needle is 
necessarily repelled as strongly as the other is attracted, since it appears that 
their relative intensity is not always the same. It therefore seems most reason- 
able to refer the phenomena of the earth’s magnetism to the agency of elec- 
trical currents existing under its surface, as well as above it : indeed, I think 
it is impossible to doubt that the changes in the intensity and direction of the 
needle, which are often so transitory, must be due to meteorological causes *. 
The aurora borealis which has frequently appeared more or less distinctly 
this winter, generally affected both needles daring some part, and only a part, 
of the time of its being visible. On the seventh of last month, the aurora was 
seen from this place as soon as it became dusk, and was still visible some time 
after midnight. It extended from N.N.E. to N.W. or W.N.W., and at intervals 
* Is it not probable that the small anomalies which have been sometimes observed in the oscillations 
of the pendulum may be owing to the same causes which produce the much more considerable irregu- 
larities in the magnetic needle ? 
f Falmouth. 
2 D 
MDCCCXXXI. 
