Natural Radiation from Moving Bodies. 579 
poke bel idyinnd Ory dy K dQ 
ine ie ae moa Ano? dt’ 
the mechanical force acting on any block or segment. of it is 
representable by pressures of intensity 
i} SS) 
Si ban Gan") de 


applied to the two ends of the segment,—these pressures just 
cancelling each other, as they “ought, when the segment 
consists ee free eee without ee The mean sting of 
this end-pressure is 
Qo" 2), 
Teal" G 
where yp and (, represent the nile of Y and (). 
When the amplitudes are diminished owing to gradual 
absorption as the disturbance travels onward, there is thus 
steady mechanical force exerted in the medium in the direc- 
tion of propagation. When the electric disturbance is inci- 
dent on a transparent reflector there is no resultant force on 
the reflecting surface itself, because y and Q both remain 
continuous in crossing it. When, however, the reflector is 
nearly perfectly opaque, the electric forces in front of it in 
the incident and reflected disturbances almost cancel each 
other, while the magnetic force just outside is doubled by its 
presence: there must thus be disturbance of the nature of 
alternating electric flux in the skin-layer of the reflector such 
as will annul this magnetic field in its interior, and it is the 
electrodynamic forces acting on this layer of current that 
constitute the aggregate electric pressure, which can be 
shown * to agree w ith Maxwell’s formula. 
From this \ way of considering the mechanical force, it is 
readily verified that when the incidence on the reflector is 
oblique, Poynting is right in taking the incident and reflected 
waye-trains each to exert their full oblique thrust on the 
reflector along their directions of propagation. 
For radiation to exert steady non-alternating pressure on a 
small body, it must t be of opaque material. <A dielectric 
mass constituted of perfectly elastic elementary vibrators 
should not be repelled by radiation. In illustration, consider 
the simplest type of vibrator, an electric doublet consisting 
of charges +e and —e separated by a varying distance J, 
parallel to x, so that its moment-M is el. When it is sub- 
jected to a ’ simple wave-train travelling along # with 
* Loe. eit. p. 133: T Loc. cit, 
2R2 
