249 



X. Supplement to a Paper " On the mutual Action of permanent Magnets, 

 considered chiefly in Reference to their best relative Position in an 

 Observatory." By the Rev. Humphrey Lloyd, D.D., Fellow of Trinity 

 College, and Professor of Natural Philosophy in the University of Dublin, 

 F.R.S., V.P.R.I.A., Honorary Member of the American Philosophical 

 Society. 



Read April 26, 1841. 



In a former paper I have investigated the conditions of equilibrium of the 

 forces exerted upon one another by three magnets, such as those employed in 

 the Dublin Magnetical Observatory, and in the Observatories since established 

 by the British government, in observing the three elements* of the Earth's 

 Magnetic Force. The axes of these magnets being supposed to lie in the same 

 horizontal plane, the forces which they exert upon one another are necessarily 

 directed in that plane ; and the conditions of equilibrium of these forces are 

 expressed hy five equations, the forces exerted upon one of the magnets, in the 

 direction perpendicular to its axis, being destroyed by the reaction of its sup- 

 ports. To fulfil these conditions, there are only four arbitrary quantities, — 

 namely, the angles v^rhich the lines connecting the centres of the three magnets 

 make with the magnetic meridian, and the azimuth of the axis of one of the 

 magnets. Hence it followed, that complete equilibrium was not attainable, 

 except for determinate values of the relative forces of the magnets. I was, 

 therefore, compelled to select among the conditions of equilibrium, all of which 



* These elements are the declination, and the horizontal and vertical components of the force. 

 The magnets employed in observing the first and second of these elements are capable of motion in 

 the horizontal plane, the axis of the first being in the magnetic meridian, and that of the second 

 perpendicular to it ; the third magnet, being supported on knife-edges, is capable of motion only in 

 a vertical plane, and its azimuth is arbitrary. 



VOL. XIX. 2 K 



