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PROFESSOR J. H. POYNTING ON RADIATION IN THE SOLAR SYSTEM: 
Though this pressure was first deduced as a consequence of the Electromagnetic 
Theory, Bae-TOLI showed, independently, that a pressure must exist without any 
theory as to the nature of light beyond a supposition which may perhaps be put 
in the form that a surface can move through the ether, doing work on the radiation 
alone and not on the ether in which the radiation exists. Professor Larmor'-"' has 
given a proof of this pressure and has shown that it has the value assigned to it 
by Maxwell, viz., that it is numerically equal to the energy density in the incident 
wave, whatever may be the nature of the waves, so long as their energy density for 
given amplitude is inversely as the square of the wave-length. We may, m fact, 
regard a pencil of radiation as a stream of momentum, the direction of the momentum 
being the axis of the pencil. If E is the energy density of the pencil, U its velocity, 
the momentum density may be regarded as E/U. 
If the stream of radiation is being emitted by a surface, the surface is losing the 
momentum carried out with the issuing stream, and is so being pressed backwards. 
If the stream is being absorbed liy the surface, then it is gaining the momentum and 
is still being pressed backwards, the forces being in the line of propagation. 
As the expressions for the radiation pressure in various cases are probably not very 
well known, it may be convenient to state them here for use m what follows. 
Values of Radiation Pressure in Different Cases. 
If 1 sq. centim. of a full radiator is emitting energy E per second, and if Ndw is 
the energy it is emitting through a cone f/w, with axis along the normal, then 
in direction 6 its projection is cos 0, and it is emitting N cos Oder through a cone doi. 
Putting do) = 277 sin 6 d6, and integrating over the hemisphere, we have 
E 
^ N cos 6 . 277 sin 6 dO = ttN. 
If we draw a liemisphere, radius r, round the source as centre, the energy falling on 
area Pdo) is N cos 0 rlco per second, and, since the velocity is U per second, the 
energy density just outside the surface on which it falls is N cos 6/Vr^, and this is 
the rate at Avhich the momentum is being received, that is, it is the normal pressure. 
The total force on area rE/co is N cos 0dco/V. This is the momentum sent out by 
the radiating square centimetre per second through the pencil with angle d(o, m the 
direction 6, and is therefore the force on the square centimetre due to that pencil. 
Eesolving along the normal and in the surface we have 
Normal pressure = N cos^ 0 dw/JJ. 
Tangential stress = N cos 6 sin 6 derjV. 
* ‘Brit. Assoc. Report,’ 1900; ‘ Encyc. Brit.,’ vol. 32, Art. “Radiation. 
