Prof. Challis on a Theory of Magnetic Force: 71 
end of the needle is consequently urged towards the east. As 
the actions on the opposite halves of the needle are equal and in 
contrary directions, the total effect is simply a motion of rotation, 
and the north end of the needle deviates towards the west, the 
direction of the galvanic current being from south to north. 
If the direction of the current be reversed, the circular stream 
passes across the magnet from west to east, and consequently by 
the same reasoning as before, the north portion of the needle is 
urged eastward, and the south portion westward. It is clear that 
the directions of the deviations of a needle above the current are 
the opposite to those of a needle below, the direction of the 
effective parts of the circular streams being opposite. 
This directive action of the galvanic current vanishes when the 
axis of the needle is transverse to the direction of the current. 
If the current acts simultaneously with the earth’s magnetism, 
the needle must take a position intermediate to the transverse 
position and the plane of the magnetic meridian. 
All these inferences from the theory are in accordance with 
well known results of experiment. 
10. The curvilinear paths of the etherial streams in the above 
theory of the magnet, correspond to Faraday’s lines of magnetic 
force, and points of greater or less velocity correspond to points 
of greater or less magnetic intensity. 
11. The explanation of the reciprocal action of magnets on 
galvanic currents on the same principles appears to be as follows. 
In the case first supposed, the magnet being under and parallel 
to the galvanic current, let the electrode be moveable about a 
vertical axis, and the magnet be fixed. Then the moving force 
which urges the magnet when moveable, reacts upon the circular 
current, and disturbs its uniformity. It has been previously 
argued that the circular movement is necessary for maintaining 
the galvanic current, by preventing its flowing towards the axis 
of the electrode. Hence under this disturbance there will be a 
tendency of the fluid to rush to the parts towards which the flow 
of the circular stream is impeded by the partial interruption, that 
is, to the west side of the north portion of the electrode. This 
impetus, being unopposed, will cause the electrode to deviate 
towards the east. Like considerations would show that the 
south portion of the electrode is made to deviate towards the. 
west. ‘Thus the magnet and electrode will have the same relative 
positions as when the former was moveable. Similar explanations 
may be given in the other cases. In general, it may be said that, 
as the galvanic current always tends to maintain itself by a 
uniform circular motion about the axis of the electrode, it tends 
also to impress on the electrode a movement by which the 
uniformity of the circular motion, when interrupted, may be 
most readily restored. 
