584 Prof. J. Larmor on the Intensity of the 
Now the flux of energy in the ether is by Poynting’s rule 
the vector ? 
—c*(hb—ge, fe—ha, ga—fb), 
so that the last term in EH, is 2c-? times the scalar product 
of this flux and the translatory velocity of the system. 
Thus the density of the radiation that is travelling in the 
enclosure in directions inclined towards v is increased ; but 
in the opposite directions it is diminished by equal amount, 
so that the aggregate density is unaltered, as already seen. 
Taking a particular case, for a plane wave-train repre- 
sented by (jf, g, h) and (a, b, c), forming part of the steady 
radiation, which thus travels in the direction perpendicular 
to both these vectors, the flux of energy per unit time is 
increased for the moving material system by a fraction of 
itself equal to twice the component of v along its direction of 
propagation divided by the velocity of light. There is dimi- 
nution in the flux for waves coming from the receding parts 
of the boundary of the enclosure, and an equal increase for 
those reflected back, giving in all the factor 4 previously 
obtained for the change of volume-intensity on reflexion. It 
may be remarked that this mode of selected orientation of 
the steady radiation in the moving enclosure clearly satisfies 
the necessary condition that, when an aperture has been made 
any where into an outer region of steady radiation, the radia- 
tion that issues through it is the same as had been previously 
sent back from the wall at that place. 
The same results for the change in the energy flux in any 
direction may be obtained directly from the flux-formula of 
Poynting, when the modified values of the vectors in the 
moving system are inserted. The connexion between the 
two methods rests on the remark that for a plane progressive 
wave the flux per unit time is the density multiplied by the 
velocity of propagation, when there is no dispersion. 
The volume-density of radiation emitted from a perfect 
radiator in any direction thus involves a factor 1+2h, where 
k is the ratio of the velocity of the radiator in that direction 
to the velocity of light; and the pressure of this ray, exerted 
directly backward, is altered accordingly, with consequences 
considered by Poynting in the memoir already referred to. 
This result is in fact what clearly obtains if on an ultimate 
dynamical theory the energies of the vibratory motions of the 
radiating sources are not affected by the uniform translation, 
but depend only on the temperature or other physical cause, 
as Carnot’s principle requires; for the amplitude of the 
vibration communicated to ether then remains the same, but 
