262 Dr Traill on Thermo-Magnetism. 
tion of the sun’s rays on its intertropical regions, and of the po- 
lar ices, must convert the earth into a vast thermo-magnetic ap- 
paratus. From the time of the celebrated Dr Gilbert, the mag- 
netic poles of the earth have been considered as “ the collected 
powers of all the magnetic ferruginous substances” which en- 
ter into the composition of our planet : but thermo-magnetism 
teaches us the important fact, that other metals as well as iron, 
under certain circumstances, are capable of acting on the mag- 
netic needle ; and the analogy of the effect of mere position in 
rendering buildings, and other vertical objects, magnetical, 
would incline us to believe that the disturbance of the equili- 
brium of temperature, even in stony strata, may elicit some de- 
gree of magnetism. he this as it may, there can be little 
doubt, that the unequal distribution of the earth’s temperature 
must have some effect of modifying its “ magnetism ef compo- 
sition!'' Dr Traill considers the direction of the needle as the 
resultant of two forces, the magnetism of composition of the 
earth, and its thermo-magnetism. The direction of these forces 
do not appear to be the same. The evident tendency of a ther- 
mo-magnetic apparatus, is to place a needle exposed to its influ- 
ence at right angles to the axes of the apparatus, or at right 
angles to the direction in which the inequality of temperature 
is propagated. If the earth be a thermo-magnetic apparatus, 
its thermo-magnetism, in like manner, must tend to place the 
needle W. and E. 
The author endeavoured to shew, that the best established 
oscillatory movements of the needle were connected with changes 
in the earth’s temperature ; and these he adduced as proofs of 
the thermo-magnetism of our planet. He further conceived, 
that the varying declination of the needle could be more satis- 
factorily explained on thermo-magnetic principles, than on any 
other hypothesis hitherto offered. He considers the existence 
of two poles of greatest cold in either hemisphere, established by 
a comparison of actual registers of temperature, as generalised 
in the isothermal lines of Humboldt ; and history, tradition, 
and geological investigation, all concur in proving that the iso- 
thermal poles have not been stationary. The short period to 
which accurate thermometrical registers reach, prevents us ha- 
ving any certain data on which to ascertain the relative tempe- 
