20 SCIENCE PROGRESS 



Some evidence in support of these conclusions is to hand, 

 and the author proposes to continue his researches on these 

 and other points of interest, and communicate them as they 

 are completed. 



A very interesting paper on the dynamical theorem generally 

 referred to as the " Theorem of Equipartition of Energy " 

 and its bearing on modern views on Radiation and the Quantum 

 Theory appears in the same number of the Phil. Mag. It 

 is too long and too closely reasoned to allow of a satisfactory 

 summary, but any one interested in these matters will be 

 well advised to read it. The author, Dr. W. F. G. Swann, 

 exposes in a clear and lucid manner two pitfalls into which 

 careless reasoning may land the unwary when applying this 

 theorem, and incidentally combats certain views as to the 

 impossibility of reconciling the equipartition theorem with 

 the quantum hypothesis, which have almost become a matter 

 of faith with the orthodox. 



In the same number of the Phil. Mag. is printed a paper 

 by E. A. Biedermann on the energy of an electromagnetic 

 field. He draws attention to a discrepancy which exists 

 when one measures the magnetic energy density at a point 

 by means of the well-known formula H 2 /87r, according as 

 H is taken to represent the mean value of the magnetic force 

 at the point, or the instantaneous value of the magnetic force. 

 Of course no discrepancy arises when one considers the current 

 giving rise to the magnetic field as distributed uniformly 

 throughout conductors. It is when one adopts the modern 

 view of electronic currents which concentrates the current 

 in the paths of a number of discrete particles, that a difference 

 in results arises according as one or other interpretation of H 

 is adopted. The author proposes to resolve the difficulty by 

 adopting a more general formula for the magnetic energy 

 density, which may be summarised thus : H, as is well known, 

 can be obtained by taking the vector product of the electric 

 intensity at a point due to a charged particle and the velocity 

 of that charge and finding the resultant of all these vectors 

 at the point in question. The author proposes in the formula 

 for the magnetic energy density to add to H 2 the square of 

 another quantity G, which is the result of summing the scalar 

 products of electric intensit}^ and velocity. On this assump- 

 tion the author shows that no discrepancy arises, and further, 



