Mr. Christie's theory of the diurnal variation , &c. 309 
to which I have referred, Professor Cumming makes this 
remark : “ Magnetism, and that to a considerable extent, it 
appears, is excited by the unequal distribution of heat amongst 
metallic, and possibly amongst other bodies. Is it improbable 
that the diurnal variation of the needle, which follows the 
course of the sun, and therefore seems to depend upon heat, 
may result from the metals and other substances which com- 
pose the surface of the earth, being unequally heated, and 
consequently suffering a change in their magnetic influence ?” 
And in the second part of a paper detailing some thermo- 
magnetical experiments, read before the Royal Society of 
Edinburgh, Dr. Traill considers, “ that the disturbance of 
the equilibrium of the temperature of our planet, by the con- 
tinual action of the sun's rays on its intertropical regions, and 
of the polar ices, must convert the earth into a vast thermo- 
magnetic apparatus and “ that the disturbance of the equi- 
librium of temperature, even in stony strata, may elicit some 
degree of magnetism.”* I am not, however, aware that any 
thermo-magnetical experiments bearing directly on the sub- 
ject of the phenomena of terrestrial magnetism, have yet 
been published. 
By varying the original experiment, Professor Cumming, 
Society in April, 1823, but was not published for a considerable time afterwards ; 
and I was not aware of its having been read when I sent my second Paper to the 
Royal Society m 1824. 
* This interesting communication appears to have been read in February, 1824, 
but I believe it has not yet appeared in the Edinburgh Philosophical Transactions ; 
and I only know it from a short abstract in the Edinburgh Philosophical journal 
for October, 1824, with which, however, I was not acquainted, until I had made 
nearly the whole of the thermo-magnetic experiments described in the present 
Paper. 
