Natural Radiation from Moving Bodies. d81 
the heat developed per second comes out to be 
“ es o 7s eee 0 cy. 
H=ol ee as l6r 327 \pe 
Now if A, is the coefficient for the wave-train Brees 
incident from the free zether and A’ that for the wave-train 
reflected back, the continuity of Q and of w—!dQ/dz across 
the surface gives 
A,+A’= A, ~(A;— A’) = ree 
pty 
so that A=3(1+ us 0) A 
pen 
and passing again to real parts by taking moduli, the ampli- 
tude of the incident train is approximately 
L(uw—roc)tA. 

The energy incident per second is thus 
: a AGCAs sor ay 
See - HAG “ 32 Tr oe 

(o"0) 
of which the part degraded thus forms a negligible fraction 
inversely proportional to the square root of the conductivity 
The waves are thus turned back without sensible loss by 
degradation, because for an ideal good conductor the surface- 
layer is at a node of the electric force. There is superficial 
current in the conductor which gives rise to the Maxwellian 
repulsion by the agency of the magnetic field, while there is 
no sensible electric resistance, the small electric force near 
the node establishing the necessary current without production 
of heat. 
The conditions which here obtain for very high conduc- 
tivity and short waves also hold for lower conductivity and 
Jonger waves. For long heat-waves the proportionality of 
the absorbing powers of metals to the square roots of their 
specific resistances has, as is well known, been discovered by 
Hagen and Rubens, and explained in advance by Drude and 
afterwards by Planck : this observation carries the interesting 
result that the resistance-coefficients are nearly the same for 
such heat-waves as for ordinary steady currents. 
Any doubt that may be entertained as to whether radiation 
exerts a back pressure on the body that emits it, may be 
diminished by considerations of the kind hereemployed. The 
