The Rev. H. Lloyd on the mniual Action of permanent Magnets. 1 73 



tana=-f^±V^T^^* + ^; (26) 



which determines the azimuth of the line connecting the three magnets. This 

 arrangement of the magnets is represented in Fig. 5. 



This is, in many respects, a very advantageous disposition. The disturbing 

 forces exerted upon the magnet a are in complete equilibrium, so that this 

 magnet (which is that employed in absolute determinations of declination and 

 intensity) may be used as if it were insulated ; and, with respect to the magnets 

 B and c, the effect of the disturbing forces is corrected by a simple change of a 

 coefficient. As to the Observatory itself, one long and narrow room, about 

 forty-eight feet in length, and sixteen feet in breadth, will suffice ; the hearing 

 of the axis of the room, along which the three magnets are to be disposed, 

 being determined by (25, 26). The magnet a should be so far from one end 

 as to allow a space of eight or nine feet in a direction perpendicular to the mag- 

 netic meridian, on either side, for experiments of deflection ; the magnet b may 

 be close to the other end. The place of the intermediate magnet will be de- 

 termined by (24).* 



Having considered the case in which three only, of the four variables, are 

 arbitrary, it remains to examine that in which there are but two disposable 

 quantities ; the other two being either absolutely determined, or else connected 

 with the rest by given relations. 



We can satisfy, in this case, but two of the equations of equilibrium ; and 

 we shall select for that purpose (\\) and (13), which express the conditions of 

 equilibrium of the forces exerted upon the magnets a and b in the direction 

 perpendicular to the magnetic meridian. These being fulfilled, the resultant 

 action on each of these magnets is directed in the magnetic meridian itself, 

 and therefore conspires with, or directly opposes, the force of the earth. Hence 

 the mean position of the magnet a is unaltered ; and the changes of position of 



* These dimensions have reference to magnets whose directive power is about the same as in 

 those employed in the Dublin Magnetical Observatory. The magnet bars, a and b, are here of 

 the same size — each 15 inches in length, f of an inch in breadth, and -J- of an inch in thickness ; 

 they are of course magnetized, as nearly as possible, to saturation. The magnet c is 12 inches 

 in length, but much smaller than a and B in its other dimensions. 



