4 
crease of 1° Fahr. produces a change of angle amounting only 
to + 0°05; so that a=+-000015, and the relative change of 
the force of the bar = + :000029. 
“If we assume that the induced magnetism of the iron bars 
is proportional to the inducing force, the coefficient p may be 
found by inverting the bars, and observing the angles of de- 
flection in the direct and inverted positions. : For, these angles 
being denoted by w and w’, it may be readily shown that 
2 
sin w+sin zw” 
jis 
It was by this method that I originally proposed to deter- 
mine the constant of the preceding formula. The assumption 
upon which it rests is the same as that which Poisson has taken 
as the basis of his theory of induced magnetism. It is, how- 
ever, as Dr. Lamont has shown, not strictly in accordance with 
fact ; and it is therefore necessary to seek another mode of 
determining the constant. It is obvious that this quantity 
will be known, if we can alter the inducing force artificially, 
by a small but known amount, and observe the change of angle 
thereby produced. ‘This is the principle of the method de- 
vised by Dr. Lamont for the purpose; it is practised in the 
following manner. 
«¢ A magnet is placed at a considerable distance above or 
below the suspended magnet, their centres being in the same 
vertical line; and it is so arranged as to be capable of rotation 
round a horizontal axis parallel to the suspended magnet in 
its deflected position. Let this magnet be first placed verti- 
cally, in which position it exerts no direct action upon the 
suspended magnet, but only on the iron bars. Then, if 2 and 
R' denote the forces exerted by the auxiliary magnet upon 
the two bars, SsU=VR, 6U =V'R’; so that if kn denote the 
corresponding change of angle, expressed in scale-divisions of 
the instrument, we have 
VR+VR =X cosu kn. 
