802 Mr Christie's Observations on Magnetic Attraction. 
on these currents, he infers, that electric currents exist in the 
earth, in planes perpendicular to the axis of the dipping needle, 
and that the corresponding electric currents in the horizontal 
needle, perpendicular to its axis, are guided by these, so that 
the needle places itself in such a position, that the planes of 
these currents are as nearly parallel as the force of gravity, 
which acts on the needle, to retain it in a horizontal position, will 
admit ; that is, that the axis of the horizontal needle will be in 
the plane perpendicular to the planes of the electric currents in 
the earth. If, now, the imaginary needle, or column of magne- 
tic particles, which I have supposed to be influenced by the 
ball, and to guide the horizontal needle, consist of circular cur- 
rents perpendicular to its axis, and the ball act principally on 
these, urging each of them to assume a position parallel to the 
tangent plane, at the nearest point of the sphere or perpendicu- 
lar to the line joining the centres of the sphere and circular cur- 
rent, so that, by the joint action of the ball and the earth, they 
assume an intermediate position, then a needle freely suspended 
by its centre of gravity, would assume such a position, that the 
tendencies of the currents perpendicular to its axis, to become 
parallel to the terrestrial electric currents in the imaginary 
needle, should be equal on each side of the centre of suspen- 
sion and these terrestrial electric currents towards the end of 
the imaginary needle, nearest to the ball, being more affected 
than those at the other, that end of the suspended needle would 
be urged towards the ball, and, consequently, the terrestrial 
currents would also guide the corresponding end of the horizon- 
tal needle towards the ball, the respective deviations being esti- 
mated by the arcs so- S' M (fig. p. 298), precisely in the manner 
which I have stated in the preceding abstract. The nature of 
this action of the ball on all the terrestrial electric currents, 
would not be unaptly represented by its action on one of them, 
that which passes through the centre of the needle, or, which is 
the same thing, on the equator of the dipping needle; and its de- 
viation, arising from this action, or the deviation of its axis, 
which is equal to it, would represent the deviation of the imagi- 
nary needle towards the ball. 
R. M. Academy, i S. Wo Christie* 
Woolwich, May 19- 1823. J 
