Intelligence and Miscellaneous Articles. 915 



the aether, but also as a very special case gives for the first time 

 (strange to say) the dynamical specification of a ray of light. 



At about the same time, 0. Heaviside, and a little later Hertz, 

 were engaged upon this aspect of electric transmission as an 

 elastic effect propagated from body to body across the aether, in 

 place of the older aspect of electric charges in movement, each 

 carrying its field of disturbance along with it, which, rejuvenated 

 ten years later by exact conceptions of the agency of electrons, 

 duly modified for change of acceleration, now includes the whole 

 field of view. But times not being yet ripe for that, he pursued 

 his subject in 1885 in another Phil. Trans, memoir " On the 

 conuexion betweeu electric current and the electric and mag- 

 netic inductions in the surrounding field," which tracks out the 

 relatious of a current circuit by the graphical device of the 

 motion of what are now known as Paraday tubes of force, a type 

 of explanation which is at the present time once more widely in 

 favour. 



Afterwards he broke new ground in the experimental deter- 

 mination of the constant of gravitation — the problem of weighing 

 the earth — which had been solved by Cavendish with his accus- 

 tomed genius by aid of Michell's principle of balancing by torsion. 

 To Poynting' s mind an ordinary balance with lever and scale-pans 

 gave at least equal promise of practical accuracy ; and his long con- 

 tinued experimental investigations, which were summed up in 

 a Phil. Trans. Memoir of 189:2 and a Cambridge Adams Prize 

 Essay of 1893, were the starting-poiut of a new interest in this 

 subject which opened up into many methods more or less cognate 

 to his own. By this time, however, the torsion method had 

 renewed its power through the discovery of the production and 

 properties of quartz fibres by C. V. Boys, whose remarkable sub- 

 sequent investigation with small-scale apparatus was generously 

 acknowledged by Poynting as the last word on the subject. 



The resource thus acquired in refined dynamical experimenting 

 was to reap further successes in a more untilled field. The ancient 

 idea of a pressure exerted by light, so obvious on the corpuscular 

 view of optics, had been revived by Maxwell on a foundation of 

 an accompanying electric stress in the transmitting medium. Its 

 mere existence, as distinct from an analysis of its propagation to 

 the place where it is in evidence, was already indubitably involved 

 in the Amperean forces on the electric currents induced in the 

 surface of a reflector. It had assumed some importance in its 

 application, notably by FitzGerald, to the elucidation of the 

 mysterious phenomena of comets' tail?. To Poynting, this pres- 

 sure exerted by a ray coming say from a distant star, far out of 

 reach of dynamical effect, involved that the ray carried momentum 

 along with it, and that the pressure effect was of the nature of a 

 thrust exerted along the ray arising from the transfer of this 

 momentum. After long efforts, the disturbances arising from 

 gaseous convection as a whole, and from the radiometric molecular 

 effect were eliminated by Lebedew, and compensated by balancing 



