343 
diurnal variation of the magnetic needle, &c. 
and in some they differ sensibly more widely, than when the 
intensity of the easterly pole was less. Likewise the variation 
from the morning maximum east to the maximum west, com¬ 
pared with the variation from the maximum west to the 
evening maximum east, does not so nearly agree with the 
ratio of the observed variations in London as in the former 
case. So that, an increase of intensity in the eastern magnet 
causes a greater discordance between the experimental re¬ 
sults and the observations. By a small diminution in the 
intensity of this pole they might agree more nearly ; but if 
it were removed altogether, that is, if we were to suppose 
only one pole to revolve in the equator, the evening maximum 
east would not obtain, with the latitude and variation corres¬ 
ponding to those of London, and the discordance in the results 
would be still greater. 
I have not as yet noticed what may appear an incongruity 
in these experimental results, as compared with the observa¬ 
tions in London and at Port Bowen, viz., that here the devia¬ 
tions corresponding to those in London are considerably 
greater than those corresponding to the observations at Port 
Bowen, whereas the diurnal changes at the latter place 
greatly exceed those at the former. This incongruity is 
however only apparent, and its cause is easily pointed out. 
The changes in the direction of the horizontal needle are 
dependant on those which would take place, under the same 
circumstances, in the direction of a needle freely suspended 
by its centre of gravity, in such a manner that, if we conceive 
their centres to coincide, and the disturbing force to act upon 
the dipping needle alone, the horizontal needle would always 
be found in the vertical plane passing through this imaginary 
