56o 



NATURE 



[December 30, 1920 



was not published until 1903, he had then held the 

 views expressed in the following quotation for a 

 long time : — 



" I have no doubt whatever that our ultimate 

 aim must be to describe the sensible in terms of 

 the sensible. But 1 see, too, what gulfs there are 

 still separating one part of our knowledge from 

 another, and I see no harm in throwing tem- 

 porary bridges of hypotheses across these de- 

 tached gulfs to connect what would otherwise be 

 detached regions. They allow us to pass to and 

 fro with ease, and have been, and are, of enor- 

 mous help to us in our exploration of Nature. 

 But we must bear in mind that we may have many 

 types of connecting bridge, many forms of hypo- 

 thesis equally serviceable, all perhaps to be 

 broken down and abandoned when we have filled 

 in the gulfs which they crossed, and have made 

 firm roadways built of sensible fact." 



As another illustration of the modernness of his 

 point of view, I may take a sentence from his 

 address as president of Section A at the meeting 

 of the British Association at Dover in 1899 : — 



"Another illustration of the illegitimate use of 

 our hypothesis as it appears to me is the attempt 

 to find in the ether a fixed datum for the measure- 

 ment of material velocities and accelerations, a 

 something in which we can draw our co-ordinate 

 axes so that they will never turn or bend. . . . 

 We could only fix positions and directions in the 

 ether by buoying them with matter. We know 

 nothing of the ether except by its effects on 

 matter, and, after all, it would be the material 

 buoys which would fix the position, and not the 

 ether in which they float." 



Poynting's scientific work concentrated in the 

 main on three subjects : the flow of energy in the 

 electromagnetic field, researches on gravity, and 

 on the pressure of light. It is by the first of these 

 that he is perhaps most widely known, and with 

 which "Poynting's vector" will always associate 

 his name. It was Poynting who introduced the 

 idea that the energy in the electromagnetic field 

 flows in the direction of a vector now known as 

 "Poynting's vector," which is at right angles to 

 both the electric and the magnetic forces, the rate 

 of flow being proportional to the product of these 

 two forces and the sine of the angle between 

 them. Thus, whenever there are both electric and 

 magnetic forces, there is flow of energv unless 

 the two forces are in tlie same direction, and 

 wherever there is flow of energy there are both 

 electric and magnetic forces. 



The importance we attach to this result will 

 depend upon the view we take of the localisation 

 and identification of energy. If we confine our- 

 selves to the dynamics of a system of bodies 

 acted upon by assigned forces, we may regard 

 the kinetic and potential energies of the system 

 NO. 2670, VOL. 106] 



as names for certain functions of the velocities and 

 co-ordinates of the bodies which satisfy a suffi- 

 cient number of equations to enable the co- 

 ordinates of the system to be determined at any 

 time if the co-ordinates and velocities at some 

 previous time are known. The idea of localisa- 

 tion of energy is foreign to this point of view, 

 for the amount of kinetic energy possessed by any 

 member of the system will dejjend upon the 

 choice of axes to which the velocities are referred, 

 while when we regard the potential energy of two 

 charges separated by a distance r as — ee'/r, the 

 idea of localisation of energy is unmeaning and 

 immaterial. When, however, we discard the idea 

 of action at a distance, and regard the space 

 between bodies as the seat of the influences they 

 exert on each other, the question of the localisa- 

 tion of energy at once becomes prominent. Thus, 

 to take a definite case, v»e may regard all energy 

 as molecular in structure, and made up of a large 

 number of units, all the units possessing the same 

 amount of energy, and the energy of each unit 

 remaining unchanged as the unit moves about. 

 Thus the amount of energy in any region is pro- 

 portional to the number of units of energy in that 

 region, and changes in this amount are due to 

 the motion into or out of this region of units of 

 energy. From these points of view the motion of 

 these units is what governs the behaviour of the 

 system, and the flow of these units is represented 

 by Poynting's vector. Poynting's work on the 

 pressure of light and his determinations of the 

 density of the earth by an ordinary balance are 

 great tributes to his skill and insight as an ex- 

 perimenter. The method with the ordinary balance 

 is, as he himself acknowledges, inferior to that 

 employed by Boys, and yet Poynting was able 

 to get, by care and skill, a result comparable 

 in accuracy with that obtained by the better 

 method. 



Poynting's experimental skill and his power of 

 devising simple apparatus which could be made in 

 the laboratory and yet give results of the highest 

 accuracy were quite remarkable, and the simplicity 

 of the means by which he got his results was not 

 surpassed either by Stokes or by the late Lord 

 Rayleigh. 



To read through everything that a man has 

 published during forty years of scientific work, 

 extending over a period remarkable for new dis- 

 coveries and the introduction of new ideas, is a 

 severe test for his reputation ; it might be ex- 

 pected that much would seem musty and out of 

 date ; this, however, is not so with Poynting's 

 papers, for in them there is little or nothing which 

 in any way conflicts with modern ideas. 



