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 M U and M U 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 force ; 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 a term 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." 



