310 
Mr. Christie’s theory of the 
Dr. Traill, M. Be^uerel, and others, obtained many highly 
interesting results; but in all these, the metals had been 
united only at particular points, and heat or cold applied at 
the joints; and I am not aware of any experiments having 
been made with different metals symmetrically united through¬ 
out. Perhaps however the following experiment, mentioned 
in the abstract of Dr. Traill’s Paper, may be considered an 
exception : “ When a piece of metal has one of its surfaces 
applied flatly to another equal metallic plate, a thermo-mag¬ 
netic combination, is formed.” It is an exception, and one 
bearing directly on the subject, if we are to look to “ the 
disturbance of the equilibrium of temperature in stony strata,” 
as the cause of any of the phaenomena of terrestrial mag¬ 
netism. If however these phaenomena are to be attributed 
to electro-magnetism excited by heat, it appears to me more 
probable, that the disturbance of the equilibrium of temperature 
arises from the inequality of the conducting powers of the 
atmosphere, and of the land or water, with which it is every 
where in contact, or of those of the land and water, than from 
the difference in the conducting powers of the various strata 
of the earth itself. 
Thermo-magnetic phaenomena have as yet only been ex¬ 
hibited by metallic combinations, but this may have arisen 
from the smallness of the masses on which we may have 
been enabled to experiment; and it is by no means impro¬ 
bable, that, were experiments made on a large scale, these 
phaenomena might be exhibited in combinations of any sub¬ 
stances whose conducting powers differ greatly. 
Admitting then the possibility of electro-magnetism being 
excited in such combinations as the earth and its atmosphere. 
