VARIATION OF TERRESTRIAL MAGNETISM. 
509 
VII. Concluding Remarks. 
Faraday, in the year 1850, discussed the diurnal variation of the magnetic needle. 
He showed that the changes which took place during daytime could be accounted for 
by supposing two magnetic poles—namely, a North pole in the Southern hemisphere, 
and a South pole in the Northern hemisphere—to be carried round with the Sun in 
our atmosphere. A glance at fig. 13 will show that our result entirely agrees with 
Faraday’s. The proof that the principal part of the Earth’s magnetism is due to 
causes outside its surface would have been almost as complete in the year 1850 as 
it is now, if Faraday had added the remark that, if all three components of the 
variation can be completely accounted for by hypothetical changes taking place outside 
the Earth’s surface, they cannot be accounted for by changes taking place in the 
inside. 
I cannot agree, however, with Faraday in the ex23lanation which he gives of the 
variation. He imagines that the solar radiation, heating uji the air, produces a 
sufficient change in its magnetic permeability to account for the observed deflection 
of the lines of magnetic force. 
The magnetic susceptibility of oxygen at the atmospheric pressure and temjDerature 
is about ‘5, 10“’^, and for air it is smaller still. This would give the magnetic per¬ 
meability as 1'0000006. If the air was entirely removed the change of magnetic 
force would be so small that we could not detect it. I have tried in various ways to 
find how a partial removal of the atmosj^here as a magnetic medium could affect the 
needle in any appreciable way, but have failed to do so. Faraday suggests that the 
oxygen in the higher regions of the atmosphere might, owing to the greater cold, be 
much more magnetic than what we observe it to be. But, on the other hand, owing 
the smaller density, the permeability would be diminished ; so that I do not think we 
are at present justified in ascribing any material part of the daily variation to a 
change of the magnetic 2 )ermeability of air due to the heating effect of the Sun. The 
effect of the Moon suggests a tidal action as the cause, and we may inquire whether 
such a tidal action could produce the observed effects. The late Professor Balfour 
Stewart has suggested that the Earth’s magnetic force might induce electric currents 
in the convection currents which flow in the upper regions of the atmosj^here. One 
difficulty of this hypothesis was removed by an experimental investigation, by means 
of which I have proved that the air can be thrown into a sensitive state in which 
small electromotive forces will produce sensible electric currents. To bring the air 
into that sensitive state it is only necessary to send an electric current through it 
from some indejiendent source of high potential. It is very likely that the air in the 
upper regions of our atmos]Dhere is in such a sensitive state, and it is quite possible, 
therefore, that the induced electric currents suggested by Professor Balfour Stewart 
really exist. 
The symmetry of the diurnal variation in both hemispheres shows that, if it is due 
