horizontal needle when under the influence of magnets. 365 
forces being here nothing. The magnets having approached 
beyond this, there will be only two points of equilibrium, a 
point of stability at 180°, and of instability at zero. The same 
effects would manifestly be produced, if instead of the force 
of the magnets being increased by their approach towards 
the centre, the terrestrial forces were to ^ decrease, whilst the 
magnets remained at the same distances : so that the receding 
of the easterly and westerly points of stable equilibrium from 
the north, would indicate a diminution, and their approach to 
it, an increase of the terrestrial magnetic intensity. 
Let us now consider what would be the effect if the line 
in which the magnets are placed described a small angle from 
the meridian, the magnets remaining at the same distances 
from the centre ; or, which is the same thing, if the direction 
of the terrestrial forces were changed without any change 
in their intensities. We will suppose that the line in which 
the magnets are placed moves through a small angle in the 
direction of the sun's motion, which is equivalent to supposing 
that the magnetic meridian describes a small angle in the 
contrary direction. 
Taking first the case of there being only one point of stable 
equilibrium , it is evident that when it is at zero, it will move 
in a direction contrary to that of the sun’s motion, and that it 
will move in the same direction as the sun, when it is at 180°. 
When there are three points of stable equilibrium , that 
which is at 180° will still move in the direction of the sun’s 
motion, and the opposite point of instable equilibrium will- 
move in the same direction, that is, from its position at zero, 
towards the east. In consequence of the decreasing angle 
