152 REPORT—1885. 
The following values of w, v, w satisfy this condition :— 
"=O "aa 
Ret a as dz dt ace}, 
cee at § ua A it 
where » is the magnetic permeability and F, G, H are the components 
of the vector potential, or if J be the electrostatic potential 
se em 
warts z l dy a}, 
_dfay afm) 
da a} dx | dz F 
d fd are! a} {er 
ee 
dx \ dy dy 
If the values of w, v, w which satisfy these conditions be denoted by 
the (%, 7, W), (U2, Vg, W2) . . . then the flow across any element of 
surface might have for its 2 component— 
ye BB - V7) + Aim + Xa te + As ty + nated 
where A,, Ag, Az are arbitrary constants, thus we see that the components 
of the flow of energy, instead of being uniquely determined by this pro- 
cess are really left quite indeterminate by it. Though thisis so, it is very 
instructive to follow Professor Poynting’s description of the way in which 
the energy flows in some special cases; we shall select a very simple one, 
the case of a current flowing along a straight wire. Here the lines of 
electromotive force are straight lines parallel to the wire, the lines of 
magnetic force are circles with their centres on the wire, and their planes 
at right angles to it. Then, since according to the view expressed in 
the paper, the energy moves at right angles both to the electric and 
magnetic forces, it must in this case move radially inwards to the wire 
where it is converted into heat. The energy, instead of being supposed 
to be transmitted through the wire, is regarded as transmitted by the 
dielectric ; and though we may not regard the exact law of flow of 
the energy as established, still it is very important that this view 
should be brought into prominence. Another important point brought 
prominently forward in this paper is the view that magnetic force is 
always the sign of transference of energy, according to Professor 
Poynting ; indeed, there must be transference of energy from-one part of 
the field to another to give rise to magnetic force. Thus, according to 
his view, no magnetic force would be exerted by the discharge of a leaky 
condenser, because in this case he considers the energy to be confined to 
the space between the plates of the condenser and to be converted into 
heat where it stands. If the plates were connected by a metallic wire, 
the energy could flow out and be converted into heat in the wire and 
this motion of energy would give rise to magnetic forces, so that magnetic 
