2 
alterations, I deem it right to resume the subject, and to lay 
the method in its complete and amended form before the 
Society. 
«¢ When a bar of soft iron is held in any direction not perpen- 
dicular to that of the earth’s magnetic force, it becomes a tem- 
porary magnet, by the inducing action of that part of the 
force which acts in its direction. The small changes of the 
induced magnetism may be assumed to be proportional to 
those of the inducing force; and, as the former may be mea- 
sured by their effects, the latter become known. 
‘To apply this simple principle to the determination of the 
variations of the vertical component of the earth’s magnetic 
force,—two soft iron bars,* of the same size and form, are to be 
placed vertically, at equal distances on either side of a small 
freely-suspended horizontal magnet, and so that the plane con- 
taining them may pass through the centre of the magnet, and 
be perpendicular to its axis. Then, if the upper extremity of one 
of the bars, and the lower extremity of the other, be in (or 
near) the horizontal plane containing the suspended magnet, 
it is obvious that they will conspire to deflect it, the predomi- 
nant pole being in one a north, and in the other a south pole. 
«<The moment of free magnetism of the suspended magnet 
being denoted by M, let MU and MU’ be the moments of 
the forces exerted upon it by the two bars. The quantities 
U and U’ are functions of the vertical component of the 
earth’s magnetic foree; and depend also upon the quantity 
and distribution of magnetism in the bars, and upon their 
position with respect to the suspended magnet. They may 
likewise each contain aterm dependent on the permanent mag- 
netism of the bars, which is seldom wholly evanescent. These 
forces conspire to turn the magnet, and are resisted by the 
horizontal component of the earth’s magnetic force, whose 
* « The employment of a second bar originated with Dr. Lamont, of Mu- 
nich, to whom (as will presently be stated) this method is indebted also for 
other improvements.” 
