172 M. "W. Siemens on the Admissibility of the 
continent, which mostly has a rocky bottom only thinly covered 
with badly conducting earth. The accumulation of the in- 
duced electricity will therefore have to be sought rather on 
the surface of the incandescent well-conducting interior of the 
earth, by the greater distance of which from the outer surface 
the preponderant influence of the masses of electricity in 
convective motion lying nearest is diminished. Whether it 
will be possible to deduce the existing distribution of the earth's 
magnetism, as well as the observed periodic and irregular 
disturbances of it, from this theory of the cause of the 
phenomenon of terrestrial magnetism, must be decided by- 
subsequent special investigation. The daily regular disturb- 
ances might be accounted for by the fact that the density of 
the induced electricity on the side turned away from must be 
somewhat less than that on the side turned towards the sun. 
This unequal density of the earth's electricity depending on 
the position of the sun, must proceed pari j)assn with the 
rotation of the earth, and may therefore be the cause of the 
regular equatorial earth-currents discussed by Lamont. The 
magnetic disturbances produced by the moon may likewise 
find their explanation in the reaction of lunar electricity upon 
the distribution of the induced electricity of the earth. On 
the other hand, the secular alteration of the situation of the 
magnetic pole can, in all probability, only be referred to yet 
unrecognized cosmic causes. 
Although this theory may still leave much unexplained, it 
yet affords at least the possibility of giving an explanation of 
the origination of the earth's magnetism fitting in with our 
present experience. This is not the case with any previous 
theory. The hypothesis of a central magnet in the interior 
of the earth is contradicted by the universal experience that 
a red heat destroys the magnetism of all bodies. Hence that 
hypothesis cannot be maintained without entirely losing sight 
of the foundation of experience. The hypothesis of a stratum 
of magnetic ore in the crust of the earth as the seat of 
terrestrial magnetism is contradicted, in the first place, by 
calculation, since the magnetism of such a stratum, even if it 
be supposed of the greatest possible thickness and magnetized 
to the maximum, would not suffice to produce the existing 
terrestrial magnetism ; and, secondly, by the impossibility of 
finding a cause for the magnetization of that stratum of ore, 
since the magnetism cannot have been present from the 
beginning, but must have first arisen after the cooling of the 
earth. 
The same might be maintained of the theory advanced 
after Faraday's discovery of the magnetic properties of the 
