September 7, 191 1] 



NATURE 



3i' 



MODERN ELECTROMAGNETIC THEORY. 

 Outlines of the Theory of Electromagnetism : a 

 Series of Lectures delivered before the Calcutta 

 University. By Dr. G. T. Walker, F.R.S. Pp. 

 viii + 52. (Cambridge: University Press, 1910.) 

 Price 35. net. 



THESE lectures were intended to present some of 

 the more important developments of electro- 

 magnetic theory in a connected and convenient form 

 for the use of advanced students in the University 

 of Calcutta and of the lecturers in outlying colleges. 

 Following the example of Abraham and Foppl's ex- 

 cellent treatise, the author prefixes a chapter on the 

 notation and methods of vector analysis, employing 

 clarendon type for vector quantities. These are 

 adhered to closely throughout the book, with the 

 slight excursions into Cartesian method which seem 

 to be inevitable. The second chapter consists of some 

 illustrations of the application of these methods to 

 the magnetostatic field. This is followed by an 

 account of the Hertzian form of the equations of the 

 electromagnetic field both for stationary and for mov- 

 ing bodies, with an indication of instances in which 

 they fail to agree with experiment, and, finally, the 

 electron theory of Lorentz is expounded so far as the 

 general equations of the field are concerned. 



The book is welcome as partly supplying the great 

 need of an English text-book setting forth clearly the 

 present state of electromagnetic theory, which seems 

 in the last few years to have emerged a little from 

 the purely tentative stage. The accumulated evidence 

 against the possibility of the determination of a 

 unique velocity of matter relative to the aether is 

 giving physicists a prepossession in favour of the 

 Lorentz field-equations. The present work seems to 

 be directed mainly towards familiarising a larger 

 public with the theory on which these equations are 

 based, as contrasted with the earlier theory of Hertz. 

 The account given is concise and free from digres- 

 sions, and well adapted to the author's purpose. 



In one respect the book might have been improved, 

 and have assisted more materially than it does in 

 establishing a uniform usage among English writers 

 on electromagnetism. The author speaks of the 

 polarisation of the aether, polarisation of the matter, 

 and of total polarisation. Now that the theory of the 

 stagnant ather is generally accepted, the time would 

 seem ripe for confining the term polarisation to the 

 second of these quantities, even if the unsatisfactory 

 term displacement has to be retained for the last, as 

 is done by Lorentz in his "Theory of Electrons" 

 (Leipzig, 1909). It would be a great advantage to 

 students beginning to read the subject if English- 

 speaking physicists would adopt a terminology 

 analogous to that used in recent German accounts of 

 the subject (e.g. Lorentz, " Enzyk der Math. Wiss." ; 

 Abraham, "Theorie der Elektrizitat," Bd. 2), where 

 the term Erregung is used for both displacement 

 (electric) and induction (magnetic). 



The author is slightly confusing in his use of 

 symbols, making an otherwise very clear exposition 

 more difficult to follow. Without warning, the nota- 

 tion for the magnetic induction is changed from B 

 NO. 2184, VOL. 87] 



to H, a new symbol, H,, being introduced for the 

 magnetic force, and an equally novel one, G\ for 

 the magnetisation. Closely following one another 

 we find the three equations, dive = p, divD' = _ p, 

 div(E + D I )=p, in each of which p has a different 

 significance. An apparent desire for brevity has 

 caused the omission of explanation which was prob- 

 ably present in the spoken lectures and would 

 make the book much easier to read for one to whom 

 the subject was unfamiliar. The process of finding 

 the average electric force over a small element of 

 volume, so fundamental to the Lorentz theory, would 

 with advantage have received fuller explanation, if 

 only by a bare definition of the averaged vectors. 



Save for the few exceptions referred to, the book 

 marks a distinct advance towards a text-book which 

 shall give its readers a clear outline of modern theory 

 as it is at present developed. 



ELEMENTARY PRINCIPLES OF AVIATION. 



(1) Elementary Aeronautics, or the Science and Prac- 

 tice of Aerial Machines. By A. P. Thurston. Pp. 

 vii+126. (London: Whittaker and Co., 1911.) 

 Price 3s. 6d. net. 



(2) The Principles of Aeroplane Construction. With 

 Calculations, Formulae, and 51 Diagrams. By 

 R. Kennedy. Pp. vii + 137. (London: J. and A. 

 Churchill, 1911.) Price 5s. net. 



(1) '• C* LEMENTARY AERONAUTICS" is a care- 

 -E-' ful collection of available information, 

 together with some elementary theorems in aero- 

 dynamics which are, on the whole, good. It is neces- 

 sary in considering early theories in aeronautics, that 

 the reader should be carefully informed as to the 

 limitations imposed by the initial assumptions, and 

 also of departures from usual practice. This feature 

 is not always sufficiently emphasised in the present 

 work. 



Generally speaking, frictional resistances are 

 ignored without mention, and on p. 20 a new defini- 

 tion of stream lines is given which is quite foreign 

 l to the more usual definition adopted in mathematical 

 I aerodynamics. Further, the equation used in this 

 I case is only applicable to an incompressible fluid, and 

 ' it is possible that for propellers such an assumption 

 ] may not be justifiable. 



A small point of printing is worth noting. Suffixes 

 : are not always clearly indicated, and might easily be 

 i misread for factors. An obvious case occurs at the 

 bottom of p. 5, where Pa cos o is written for P„cosa. 

 The first thirty-one pages provide a sound risume 

 of current data, accompanied by illustrations of the 

 motion of air round obstacles. The advantage of 

 aerocurves as compared with planes is clearly and 

 correctly indicated. The work then proceeds imme- 

 diately to less certain ground in the discussion of 

 stability. The formula for damping of oscillations 

 deduced on p. 37 appears to be wrong, as the damp- 

 ing effect of a plane is largely dependent on the for- 

 ward speed— an effect which is ignored. 



The chapter on propellers and helicopters is good 

 only if it be clearly borne in mind that ideal condi- 

 tions are assumed for most of the theoretical de- 



