28 
The Principles of Magnetism. 
[J anuary , 
Although we cannot see the molecules of ordinary mag- 
nets, and thus perceive their movements in response to the 
general magnetic energy, yet we possess one instance of a 
magnet whose magnetic molecules are plainly visible, and 
in which we can trace the operation of the principle just 
laid down. This great magnet is the earth. Its molecules 
are its crystals, or the other definite units of matter of which 
it is composed. In the magnetic needle we have a strongly 
magnetised terrestrial molecule, by whose aid we can test 
the conditions of the earth’s magnetism. 
In the ordinary consideration of the earth’s magnetic 
phenomena there is, what seems to the writer, a misconcep- 
tion of the true relations between the earth and the magnetic 
needle. They are treated as if they were two separate mag- 
nets, and their adtion upon each other viewed in the same 
light as we view the mutual action of two ordinary magnets. 
Yet the fadt is that the needle forms part of the earth’s 
solid surface, and its magnetism is part of the earth’s mag- 
netism. We speak of the north and south poles of the earth 
with the tacit idea that all the magnetism between these 
poles lies in the same diredtion, and that every one of the 
separate magnetic masses, whose total sum of force com- 
poses the earth’s magnetic energy, has its poles turned in 
conformity with the diredtion of these north and south poles. 
Yet the behaviour of the magnetic needle should give us a 
different idea. At the equator it turns its poles opposite to 
those of the earth. And this is not because it stands above 
the immediate surface, for if laid upon, or buried beneath 
and incorporated with this surface, it would not change its 
axial diredtion. Only when taken from the equator towards 
the poles does it gradually change this diredtion, its axis 
becoming completely reversed only when the poles are 
reached. 
In all this we have a significant indication of the 
true conditions of terrestrial magnetism. Although all 
matter is magnetic, only a small portion of the earth’s ma- 
terial is strongly so, though the weak magnetism of the 
remaining material may slightly add to the total sum of 
magnetic energy. These strongly magnetic masses are dis- 
posed at random throughout the earth’s crust, and it is their 
mutual adtion which constitutes the main sum of the earth’s 
magnetic energy. In the adtion upon the needle we cannot 
assume that some hypothetical concentration of energy at 
the north and south poles controls its movements, but must 
rather consider it as principally affedted by the vigour of the 
contiguous magnetic masses, and as assuming a position in 
