344 Prof. Maxwell on the Theory of Molecular Vortices 
motion relative toit. It is evident that, from this figure, we can 
trace the variations of form of an element of the fluid, as the 
form of the element depends, not on the absolute motion of the 
whole system, but on the relative motion of its parts. 
In front of the wire, that is, on its east side, it will be seen that 
as the wire approaches each portion of the medium, that portion 
is more and more compressed in the direction from east to west, 
and extended in the direction from north to south; and since 
the axes of the vortices lie in the north and south direction, their 
velocity will continually tend to increase by Prop. X., unless 
prevented. or checked by electromotive forces acting on the cir- 
cumference of each vortex. 
We shall consider an electromotive force as positive when the 
vortices tend to move the interjacent particles upwards perpendi- 
eularly to the plane of the paper. 
The vortices appear to revolve as the hands of a watch when 
we look at them from south to north; so that each vortex moves 
upwards on its west side, and downwards on its east side. In 
front of the wire, therefore, where each vortex is striving to in- 
crease its velocity, the electromotive force upwards must be 
greater on its west than on its east side. There will therefore 
be a continual increase of upward electromotive force from the 
remote east, where it is zero, to the front of the moving wire, 
where the upward force will be strongest. 
Behind the wire a different action takes place. As the wire 
moves away from each successive portion of the medium, that 
portion is extended from east to west, and compressed from north 
to south, so as to tend to diminish the velocity of the vortices, 
and therefore to make the upward electromotive force greater 
on the east than on the west side of each vortex. The upward 
electromotive force will therefore increase continually from the 
remote west, where it is zero, to the back of the moving wire, 
where it will be strongest. 
It appears, therefore, that a vertical wire moving eastwards 
will experience an electromotive force tending to produce in it 
an upward current. If there. is.no conducting cireuit m_ con- 
nexion with the ends of the wire,:no.current will be formed, and 
the magnetic forces will not be altered; but if such a circuit 
exists, there will be a current, and the lines of magnetic force 
and the velocity of the vortices will be altered from their state 
previous to the motion of the wire. The change in the lines of 
force is shown in fig. 5. The vortices in front of the wire, instead 
of merely producing pressures, actually increase in velocity, while 
those behind have their velocity diminished, and those at the sices 
of the wire have the direction of their axes altered ; so that the 
final effect is to produce a force acting on the wire as a resist- 
