70 Prof. Challis on a Theory of Magnetic Force. 
galvanic currents relative to an electrode is composed of uniform 
motions parallel to the electrode, and of uniform circular motions 
about its axis. (See art. 10 of the communication to the ‘ Phi- 
losophical Magazine’ for December.) The theory I am about to 
give of the mutual action between magnets and galvanic currents, 
essentially depends on the existence of the circular motion: but 
it is remarkable that the facts to be explained require that the 
direction of this motion should be always the same; that is, if 
the electrode be parallel to the earth’s axis, and the current flow 
from south to north, the cireular motion must be in the same 
direction as that of the earth’s rotation about its axis. Ido not 
at present profess to account antecedently either for the circular 
motion, or for its having a determinate direction; but I can 
conceive that both these characteristics of the theoretical gal- 
vanic currents may be referable to the primary currents, which, 
as I shall hereafter attempt to show, have their origin im the 
earth’s rotation. 
9. Assuming galvanic currents to be such as are described 
above, suppose a straight horizontal electrode to be placed in 
the plane of the magnetic meridian, and the current to flow from 
south to north; and let a horizontal magnetic needle be placed 
directly underneath the electrode at a small distance from it. 
Since the needle and electrode are parallel, if the galvanic cur- 
rent were wholly longitudinal, there would seem to be no cause 
of disturbance of the needle, because the circumstances of the 
eether would be alike on the opposite sides of the plane of the 
magnetic meridian. But suppose the motion of the ether along 
the electrode to be accompanied by a circular motion about its 
axis in the direction from above towards the mght hand of a 
person looking northward; and calling the end of the needle 
which points to the north A and the other end B, let A designate, 
as heretofore, that end from which the magnetic streams asswe in 
curved diverging courses; also, abstract for the present from 
the longitudinal motions both galvanic and magnetic. Then 
the circular motion produces a stream which crosses the magnet 
from east to west. This stream meets the parts of the issuing 
magnetic streams resolved in the horizontal direction, on the 
east side of the north portion of the needle, and flows with them 
on the west side. There is consequently an increase of density 
and pressure on the east side, and a diminution of density and 
pressure on the west side, and the needle is consequently urged 
towards the west. About the end Band the sowth portion of the 
needle, where the entering streams converge in curved courses, 
the parts resolved horizontally conspire with the circular streams 
on the east side, and oppose them on the west side. The greater 
density of the ether is therefore on the west side, and the south 
