394 Prof. J. H. Poynting on 



Euler and used by him 160 years ago to explain the formation 

 o£ comets' tails by repulsion, seems to have dropped out of 

 sight, till Maxwell, in 1872, predicted its existence as a con- 

 sequence of his Electromagnetic Theory of Light. It is 

 remarkable that it should have been brought to the front 

 through the investigation of such a special type, such an 

 abstruse case, of wave-motion, and that it was not seen that 

 it must follow as a consequence of any wave-motion, what- 

 ever the type of wave we suppose to constitute Light. I 

 believe that the first suggestion that it is a general property 

 of waves is due to Mr. S. Tolver Preston, who in 1876 * 

 pointed out the analogy of the energy-carrying power of a 

 beam of light with the mechanical carriage by belting, and 

 calculated the pressure on the surface of the Sun by the 

 issuing radiation, obtaining a value equal to the energy- 

 density in the issuing stream, without assumption as to the 

 nature of the waves. But though the analogy is valuable, I 

 confess that Mr. Preston's reasoning does not appear to me 

 conclusive, and I think it still remains an analogy. There is, 

 I suspect, some genei al theorem yet to be discovered, which 

 shall relate directly the energy and the momentum issuing 

 from a radiating source. It seems possible that in all cases 

 of energy transfer, momentum in the direction of transfer is 

 also passed on, and therefore there is a back pressure on the 

 source. Such pressure certainly exists in material transfer, 

 as in the corpuscular theory. It exists too, as we now know, 

 in all wave transfer. From the investigation below (p. 397) 

 it appears to exist when energy is transferred along a re- 

 volving twisted shaft. In heat-conduction in gases, the 

 kinetic theory requires a carriage of momentum from hotter 

 to colder parts ; so that there is some ground for supposing 

 the pressure to exist in all cases. 



Though we have not yet a general and direct dynamical 

 theorem accounting for radiation pressure, Professor Larmorf 

 has given us a simple and most excellent indirect mode of 

 proving the existence of the pressure, which applies to all 

 waves in which the average energy-density for a given 

 amplitude is inversely as the square of the wave-length. Let 

 us suppose that a train of waves is incident normally on 

 a perfectly reflecting surface. Then, whether the reflecting 

 surface is at rest, or is moving to or from the source, 

 the perfect reflexion requires that the disturbance at its 

 surface shall be annulled by the superposition of the direct 

 and reflected trains. The two trains must therefore have 



* Engineering, 1876, vol. xxi. p. 83. 

 f Encyc. Brit, xxxii. Radiation, p. 121. 



