@ Theory of the Earth, — a9 
was clearly feen by M. Zpinus, that this magnet, if it-ex- 
ifts, is funk to too great a depth in the earth for the action 
of the felar rays, or at leaft the variations of that action, in 
the morning and evening, to be able to penetrate to it. We 
4 “may, however, apply to the ferruginous minerals, difperfed 
in abundanete over the furface of the earth, what Mr. Canton 
fuppofed, in regard to the grand magnet contained in its 
bofom; and then, if we admit that thefe minerals exercife 
any action on the magnetic needle, we cannot deny that the 
“heat, excited by the rays of the fun, may diminifh that ac- 
tion. From thefe principles it would follow, that in the 
morning, when the fun warms the furface of the ground fitu- 
ated to the eaft of the needle, the latter, being lefs ftrongly 
attracted towards that part, ought to decline towards the 
weft; and, for a contrary reafon, it muft in -the evening de- 
cline towards the’ eaft. But Mr. Canton proved, by a long 
feries of obfervations, that at London, at leaft, this is the 
ordinary courfe of the diurnal variations. 
7. But it willbe proper to examine whether this explana- 
tion, even thus corrected, does not contain a paralogifm ; 
and when the attraétive force of all the ferruginous particles, 
difperfed over the furface of the earth, to the eaft of the needle, 
is diminifhed equally and fimultaneoufly, the needle ought 
not to remain motionlefs; fince the diminution of the attrac- 
tion exercifed upon the fouth-pole of the needle, compenfates 
for the diminution of that exercifed upon the north-pole*, 
* Let © be the centre of fufpenfion of the needle N.S., and a, 4, c,d, 
the forces which attraét the needle in oppofite direc- 
_tions; for example, pieces of iron. The forces in 
and dconfpire to make the extremity N of the Eaft. 
needle move towards the weft; and theforces ata ..% i 
hea iabiat: hep et owe 
and ¢ con{pire in the like manner to make the fame 4 4 
exttemity proceed towards the eaft. When the needle Wer. 
Temains at reft, there is an equilibrium, and the 
forees a 4-¢ = 44d, But in this fuppofition, if 
the forces of the fame fide, 4 and c for example, gradually diminith, th 
equilibrium will not be interrupted. For, letS—=y + m andc=x +m: 
if the forces 4 and ¢ are eyually diminithed by the quantity , we fhall 
always havea+2—=6+4y. The cafe will be the fame with any aug 
Mentaiion, if it be equal and Gmultaneous on all the fides of the needle. 
*@—Nore of the Author, 
Es I fay 
