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265 
magnet will be undisturbed by the second, so as to give the 
absolute declination truly; and, as to the variations of the 
declination, it is manifest that they will be thereby increased 
or diminished in a given ratio; so that the true variations 
will be obtained by simply altering the coefficient of the 
scale. When the above-mentioned condition is introduced 
into the equation which determines the direction of the re- 
sultant force exerted by one magnet on another, (the length 
of the magnets being supposed small in comparison with 
the distance between them,) we find, for the azimuth of the 
line connecting the two magnets, referred to the magnetic 
meridian, 
1 
are (tan = V3) = 35° 16 
This result has been already obtained by Gauss and Weber. 
It is manifest that, in this case, the action of the first 
magnet on the second will not take place, either in the mag- 
netic meridian, or in the plane perpendicular to it; so that 
the second magnet is necessarily disturbed. With two mag- 
nets, accordingly, it is impossible to avoid the effects of 
mutual action. The case is different, however, when a 
third magnet is introduced. It is then possible to annul 
completely all action, with the exception of that exerted 
on the third magnet by the first and second; and this, in 
the case under consideration, is destroyed by the nature of 
the suspension. 
The third magnet about to be employed in the Dublin 
Observatory is intended for the observation of the vertical 
component of the. magnetic force. It is a bar supported on 
knife-edges, capable of motion in a vertical plane, and 
brought into the horizontal position by means of a weight. 
The three instruments being in the same horizontal plane, 
it is manifest that the action of the first and second on the 
third must take place in that plane; and this action can 
have no effect in disturbing the magnet, its motion being 
