137 
a Theory of the 'Earth. 
3. It must then be examined, whether the direction of 
the magnetic needle, and its inclination, depend on the 
situation of a large magnet enclosed in the bowels of the 
earth, as Halley supposes ; or on the action of one of 
these two magnetic fluids towards one of the poles, and, 
perhaps, of the other fluid towards the opposite pole, as M. 
Prevost supposes. 
1. If we admit the hypothesis of a large magnet sus- 
pended within the cavity of the earth, shall we suppose, 
as the inventor of this hypothesis, that this magnet has 
four poles ? Or, shall we endeavour to explain the whole, 
as that great geometer Euler has done, by a magnet hav- 
ing only two poles ? Or, lastly, shall we suppose, as Mr. 
Churchman, an American philosopher, has lately done, 
that the earth contains two magnetic poles, one at the 
north and the other at the south at different distances 
from the poles of the earth, which perform their revolu- 
tions in different times ; and that, from the combined in- 
fluence of these two poles, we may conclude the annual 
changes of the declination with so much precision, that 
we can deduce the longitude of any place from its lati- 
tude, and from the degree of declination which the needle 
experiences.^ 
3. Thus, by supposing one or more magnets in the 
interior part of the earth, the annua] changes of the de> 
eli nation and inclination may be explained by the rotary 
freely suspended towards the north, are equal to those which attract the opposite 
pole towards the south. Coulomb concludes that this equality exists, because a 
needle, weighed two different times, before and after it was magnetised, was found 
exactly of the same weight. 
C. Borda has found, by observations made first at Brest, Cadiz, Teneriffe, 
Goree on the coast of Africa, and afterwards at Brest and Guadaloupe, that the 
intensity of the force exercised by the globe on the magnetic needle, estimated 
according to the number of oscillations made by the needle in a given time, was 
sensibly the same in these different places. This observation in other latitudes, 
especially on approaching the poles, might throw some light on the theory of 
natural magnetism.— Note of C Haby. 
* Heads of Lectures by 8. Priestley, London 1794 
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