146 Lovering on Magnetic Observations at Cambridge. 
proportional to the energy of the inducing cause, that is, to the 
vertical component of the earth’s force; while the counteracting 
force is the horizontal component of the same force, acting directly 
on the magnet itself, to bring it back to the magnetic meridian. 
Thus the magnet will take up a position of equilibrium, under 
the action of these opposing forces; and this position will serve 
to determine the ratio which subsists between them. When the 
right line connecting the centre of the horizontal magnet, and the 
acting pole of the bar, is perpendicular to the magnetic meridian, 
the tangent of the angle of deflection will measure the ratio of 
the two forces, and will therefore be proportional to the tangent 
of the magnetic inclination. Accordingly, by observing the changes 
of position of the horizontal magnet, so circumstanced, we can 
infer those of the inclination itself. 
“But the iron bar may have (and generally will have) a cer- 
tain portion of permanent magnetism, which will concur with the 
induced magnetism in producing the deflection; and it becomes 
necessary to institute the observations in such a manner as to 
be able to eliminate the effects of this extraneous cause. For 
this purpose we have only to invert the bar, so that the acting 
pole, which was uppermost in one part of the observation, shall 
be lowermost in the other. ‘The induced polarity will, under 
these circumstances, be opposite in the two cases; and the 
acting force will, in one case, be the sum of the induced and 
permanent forces, and in the other their difference.” 
In another communication made soon after, Professor Lloyd as- 
sures the Irish Academy of his confidence in his new instrument 
for observing changes of inclination, though he is distrustful of its 
value for giving absolute results. He justly remarks, that sources 
of error or disturbance, which mutually compensate when an 
