a Theory of the Earth . 450 
magnetic needle, we cannot deny that the heat, excited by 
the rays of the sun, may diminish that action. From 
these principles it would follow, that in the morning, 
when the sun warms the surface of the ground situated to 
the east of the needle, the latter, being less strongly at- 
tracted towards that part, ought to decline towards the 
west ; and, for a contrary reason, it must in the evening 
decline towards the east. But Mr. Canton proved, by a 
long series of observations, that at London, at least, this 
is the ordinary course of the diurnal variations. 
7* But it will be proper to examine whether this ex- 
planation, even thus corrected, does not contain a paralo- 
gism ; and when the attractive force of all the ferruginous 
particles, dispersed over the surface of the earth, to the 
east of the needle, is diminished equally and simultaneous- 
ly, the needle ought not to remain motionless ; since the 
diminution of the attraction exercised upon tlie^south-pole 
of the needle, compensates for the diminution of that ex- 
ercised upon the north-pole.* I say the same thing of 
those situated to the west. If this reasoning be just, the 
needle ought not to vary by the action of the solar heat, 
but when this heat diminishes the magnetic force of the 
ferruginous parts situated to the north of the needle, more 
than that of the parts situated to the south, or reciprocally. 
* Let O be the centre of suspension of the needle N. S., and a 3 b } c, d, the for- 
ces which attract the needle in opposite directions ; for 
example, pieces of iron. The forces in b and d con- 
spire to make the extremity N of the needle move 
towards the west ; and the forces at a and c conspire in 
the like manner to make the same extremity proceed 
towards the east. When the needle remains at rest, 
there is an equilibrium, and the forces a-\-c=b-\-d. But 
in this supposition, if the forces of the same side, b and c 
for example, gradually diminish, the equilibrium will not be interrupted. For, 
let b=y-\-m and c~z-\-m : if the forces b and c are equally diminished by the 
quantity m, we shall always have a-\-z~b-\-y. The case will be the same with 
any augmentation, if it be equal and simultaneous on all the sides of the needle 
'—Note of the Author. 
East. 
a d 
N )- — O ( S 
b c 
West. 
