64 Professor CuMMING on the Developement 
the effect was not increased by using two such wires at the same 
time; but a silver wire of half the diameter was, in consequence of 
its superior conducting power, at least equally efficacious, (Table X.) 
These experiments are confessedly given as mere appreximations, 
which, hereafter, I hope to rectify, by the aid of a more powerful 
and accurate apparatus. 
The new subjects and modes of experiment arising from this 
hitherto untried department of science, and the light it promises 
to throw upon all enquiries connected with heat and electricity, 
have, I fear, led me to encroach too much upon the indulgence 
of those who may not have the same inducement as myself to excite 
their attention: I shall therefore conclude, with one quzre, which 
suggested itself to me whilst writing this Paper, and which, whether 
true or false, seems to have at least as much plausibility, as some 
theories that haye been recently advanced upon this subject. 
Magnetism, and that to a considerable extent, it appears is ex- 
cited 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 compose the surface of the earth, being 
unequally heated, and consequently suffermg a change in their 
magnetic influence ? 
