diurnal variation of the magnetic needle , &c. 327 
usual ways, they will likewise be exhibited when the line of 
junction is symmetrical throughout, as with a ring of one 
substance surrounding a circular plate of another, heat being 
any where applied ; and that these phenomena will nearly 
correspond to a polarising of the compound disc, similar 
to that which I have indicated for the plate of bismuth 
and copper ; the precise positions of the points to be con- 
sidered as poles depending upon the nature of the substances 
united. 
Admitting, then, that the earth and the atmosphere are sub- 
tsances in which such action can, under any circumstances, 
take place, these experiments would indicate that any portion 
of the earth, hounded by parallel pla?ies, with the atmosphere 
surrounding it, would become similarly polarised, if one part 
were more heated than another. Thus, considering alone 
the equatorial regions of the earth, we should have two mag- 
netic poles on the northern side, and on the southern side two 
poles similarly posited ; the poles of different names being opposed 
to each other on the contrary sides of the equator. 
In order fully to investigate this subject, it would be neces- 
sary that we should know the times of maximum and mini- 
mum heat, at least in the equatorial regions, both on the 
continents and likewise over the surface of the sea ; if how- 
ever we assume, that, in general, the greatest heat occurs 
about 3 o’clock in the afternoon, and the least about 5 o’clock 
in the morning, when the sun is vertical to the equator, 
which is probably very near the truth, it will serve to give 
an outline of the effects that would be produced on the 
needle by the revolution of such poles, developed on each 
side of the equator. According to this view, the coldest point 
