426 
MR. 0. HEAVISIDE ON THE FORCES, STRESSES, AND 
increasing its kinetic energy, is highly speculative. If it come from the ether (and 
where else can it come from ?), it should be possible to symbolise this in X, if not in 
qT; but in default of a knowledge of its distribution in the ether, we cannot do so, 
and must therefore turn the equation of continuity into 
S + conv (qT + X) = T, .(4) 
where S indicates the rate of supply of energy per unit volume from the gravitational 
source, whatever that may be. A similar form is convenient in the case of intrinsic 
stores of energy, which we have reason to believe are positioned within the element 
of volume concerned, as when heat gives rise to thermoelectric force. Then S is the 
activity of the intrinsic sources. Then again, in special applications, T is conveniently 
divisible into different kinds of energy, potential and kinetic. Energy which is 
dissipated or wasted comes under the same category, because it may either be regarded 
as stored, though irrecoverably, or passed out of existence, so far as any immediate 
useful purpose is performed. Thus we have as a standard practical form of the 
equation of continuity of energy referred to the unit volume, 
S + conv {X + q (U + T)} = Q + U + T.(5) 
where S is the energy supply from intrinsic sources, U potential energy and T kinetic 
energy of localisable kinds, q (U + T) its convective flux, Q the rate of waste of 
energy, and X the flux of energy other than convective, e.g., that due to stresses in 
the medium and representing their activity. In the electromagnetic application we 
shall see that U and T must split into two kinds, and so must X, because there is a 
flux of energy even when the medium is at rest. 
§ 5. Sometimes we meet with cases in which the flux of energy is either wholly or 
partly of a circuital character. There is nothing essentially peculiar to electromagnetic 
problems in this strange and apparently useless result. The electromagnetic instances 
are paralleled by similar instances in ordinary mechanical science, when a body is 
in motion and is also strained, especially if it be in rotation. This result is a necessary 
consequence of our ways of reckoning the activity of forces and of stresses, and serves 
to still further cast doubt upon the “ thinginess ” of energy. At the same time, the 
flux of energy is going on all around us, just as certainly as the flux of matter, and 
it is impossible to avoid the idea ; we should, therefore, make use of it and formularise 
it whenever and as long as it is found to be useful, in sjoite of the occasional failure to 
obtain readily understandable results. 
The idea of the flux of energy, apart from the conservation of energy, is by no 
means a new one. Had gravitational energy been less obscure than it is, it might 
have found explicit statement long ago. Professor Poynting* brought the principle 
* Potnting, ‘ Phil. Trans.,’ 1884. 
