916 Intelligence and Miscellaneous Articles. 



against each other by Nicholls and Hull, and the Maxwell value 

 of the direct pressure was fully verified. But Poynting's line of 

 thought led him straight to a tangential component of the thrust 

 as well as a direct component, and he noted that the former could 

 be investigated without much trouble from the gas-effect. This 

 idea led to many beautiful determinations in conjunction with his 

 assistants and students. His idea of convected momentum also 

 led him in another direction to the conclusion that the pressure 

 on a receding surface must be less than on one at rest, so that by 

 reaction a moving radiating body would be accelerated — an im- 

 possible result which is corrected by recognizing that its radiation is 

 greater towards the direction of its motion, which leads to retar- 

 dation on the whole. As the effect depends on extent of surface^ 

 it is greater in proportion for small bodies. Thus he was led to- 

 consider clouds of cosmic dust revolving round the sun, heated 

 by his rays to an equilibrium temperature of the space where 

 it is, retarded in its motion by the reaction of its own radiation,. 

 and thus gradually sucked into the sun. This clearance of solar 

 spaces from dust must be a prominent feature in views of cosmo- 

 gony. Its calculation aptly illustrates his latent mathematical 

 power, which was never unduly obtruded ; and the whole memoir 

 is an example of the simplification of reasoning and reduction to- 

 its lowest terms which is a mark of the depth of vision that 

 belongs to genius. When the theory of electrons came to be 

 developed into Maxwell's channels by Lorentz and others, it 

 appeared at once that his stress argument was in default, and 

 the natural first conclusion was against the objective existence of 

 Maxwell's stress as thus specified in favour of one too complex for 

 simple expression. But later Poincare and Abraham introduced 

 the idea of grouping the outstanding terms as a distribution of 

 electric momentum, specified very simply as the vector product of 

 the sethereal and magnetic inductions. This includes precisely the 

 momentum of radiation elucidated by Poynting. Interest in the 

 subject is thus stimulated, and the problem noAv under discussion 

 as to wmether the residue is in all cases simply momentum, and 

 whence comes the subsidiary travelling inertia which is implied in 

 such momentum, becomes of pressing dynamical interest. An 

 experimental and theoretical incursion into the different field of 

 the elongation of a wire due to its torsion was primarily suggested 

 to Poynting by these problems. 



The formulation of an exact notion of the temperature of space, 

 above indicated, is but one phase of his interest in the theory of 

 natural radiation ; and it seems but yesterday that he was dis- 

 cussing, in private correspondence, with his usual acuteness and 

 judgment and no sign of failure of powers, the theory of Stefan's 

 law and its other fundamental relations. J. L. 



