The Rev. H. Lloyd on the mutual Action of permanent Magnets. 255 



It may be useful to suggest, in a few words, the form of building adapted to 

 these arrangements. 



For the arrangement represented in Fig. 3, the ground-plan of the building 

 may be a square, whose sides (24 feet in length) are parallel and perpendicular 

 to the magnetic meridian, (Fig. 6). This area may be conveniently divided into 

 four parts, viz. : a principal room, 24 feet in length and 16 feet in width ; two 

 subordinate rooms, and a vestibule. The principal room should contain the 

 magnets a and b, which may be placed at an interval of 18 feet,* the joining 

 line being the axis of the room. Two pedestals, a' and b', (at an interval of 

 4^ feet), will serve to support the reading telescopes ; and the observer's chair 

 may be placed between them. The magnet c should be placed in one of the 

 small rooms, its distance from the magnet a being ac = ab X tang 35° 16' = 

 18 X 0.707 = 12.73 feet. In order to diminish, as far as possible, the de- 

 flecting force exerted by the magnets b and c upon a, these magnets should 

 have their poles similarly placed (i. e. the same pole in each turned to the east) ; 

 for, in this case, the resulting action is the difference of the forces exerted by 

 the separate magnets. 



It will be convenient to fix another pedestal, D, for the support of an incli- 

 nation instrument, in the second of the small rooms, and at the point corres- 

 ponding to c in the first ; — the line bd being perpendicular to . the magnetic 

 meridian, and the distance bd = ac. It is manifest that, in this position, the 

 action of the magnets b and c upon a magnetic particle at d will be perpen- 

 dicular to the magnetic meridian ; and will, therefore, have no effect upon the 

 position of the inclination needle, being destroyed by the reaction of its supports. 

 And, in order that the action of the magnet a may be in the same direction, it is 

 only necessary to turn it round, so that its axis may lie in the line ax, which 

 makes with the magnetic meridian an angle bax = bad. For tan d = v'2 ; 



and tan dax = 5 z: 2 v''2 ; so that tan d = ^ tan dax, and db is the 



* At this distance, the deflection produced by the magnet b upon A, (the deflecting magnet 

 being of the size and power of those employed in the Dublin Magnetical Observatory), is only 

 about 1^ minutes ; and the greater part of this small disturbance will be annulled by the opposing 

 action of the magnet c . 



