JO"- 



NA TURE 



[August 13, 1908 



are therefore a summary in non-mathematical lan- 

 guage of the chief methods and results of hydro- 

 dynamical theory. They are, on the whole, very 

 clearly written, and present in simple form all the 

 most important points of the' theory. The author is 

 not, however, content to follow orthodox theory, but 

 deviates from it in the treatment of several problems. 

 Thus on p. 6 he introduces what he calls the " prin- 

 ciple of no momentum," i.e. that no resultant momen- 

 tum can be communicated to a fluid enclosed in a 

 rigid boundary. Ag^ainst the principle as stated no 

 objection can be taken ; but when, as on p. i6, it is 

 applied to the motion of a body in an infinite fluid, 

 it may easily lead to false results. 



The second chapter contains an investigation on 

 viscosity and skin friction. This has always been a 

 subject of contention among writers on aerodynamics. 

 Langley and Dines maintain that the tangential force 

 on a plane moving sideways through the air is 

 negligible, less than o'oi per cent, of the force at 

 normal incidence. Lilienthal, on the other hand, found 

 the tangential force to be between i per cent, and 

 2 per cent, of the force at normal incidence, and 

 therefore at small inclinations it may be comparable 

 with the normal force. Mr. Lanchester, in the second 

 chapter, brings some theoretical considerations to bear 

 on the question, and proves that the force on a plane 

 moving sideways varies as ii'-^ This result is obtained 

 by supposing the motion laminar, which seems to us 

 quite unjustifiable. In chapter .\. an account is given 

 of experiments made on gliders to obtain a value for 

 skin friction. The experiments appear to have been 

 very carefully conducted, and considering the difficul- 

 ties involved give remarkably consistent results. We 

 hope that the author may be able to extend these 

 experiments and so help to decide this question. 



In chapter iv. the author leaves behind the solid 

 ground of orthodox theory altogether, and attempts 

 to work out the motion of a curved lamina or aerofoil. 

 The importance of curved surfaces in flight was first 

 realised by Philipps and Lilienthal. It seems to us 

 that the author is wrong in claiming to be the first 

 to give a theory of the motion of curved surfaces, and 

 that Lilienthal had only a practical acquaintance with 

 the curved form, for Lilienthal clearly realised that the 

 effect of curvature was to diminish the eddy motion 

 and to give an increased upward pressure due to the 

 centrifugal force of the air. The theory has been 

 worked out mathematically by Kutta, and his results 

 are in fair agreement with Lilienthal's experiments. 

 The author of the present volume attempts to work 

 out the problem by applying the theory of cyclic 

 motion to the motion of a surface in two dimensions, 

 but it is ditTicult to see how this can have any applica- 

 tion to the case of a lamina moving in free air. The 

 next chapters contain discussion of various problems 

 connected with flight; many of them are verv in- 

 teresting, but as they turn on the relative value of 

 tangential and normal force, they are, in the present 

 uncertainty as to these values, rather premature. 



The last chapter contains an account of the experi- 

 mental work of Dines and Langley. We could have 

 wished that the author had extended this chapter to 

 NO. 2024, VOL. 78] 



include the work of Continental writers such as Lilien- 

 thal, Wellner, von Lossl, &c. , much of whose work 

 is very valuable, and buried in technical journals which 

 are inaccessible to English readers. Had he done this 

 and excluded some of the more shaky theoretical 

 chapters, the work might without exaggeration be 

 called a complete treatise on aerodynamics. 



SPECTROSCOFY. 

 Handbiich der Spectroscopie. By H. Kayser. Vol. iv. 

 Pp. xix+ii48. (Leipzig: S. Hirzel, 1908.) Price 

 72 marks. 



I^'HE amount of material available on the general 

 phenomena of absorption is well known to every- 

 one who has interested himself in this branch of 

 science, and it is impossible to withhold admiration 

 for the manner in which the subject is treated in the' 

 present volume. It is not easy in a short notice to 

 discuss in detail the merits or demerits of so varied 

 a chapter of contents as we have before us, but a brief 

 glance will serve to show the lines followed by the 

 author. 



Whereas in the latter portion of the third volume 

 there was given a list of compounds of known con- 

 stitution, together with their absorption spectra, the 

 first three chapters of the present volume contain an 

 account of the absorption of colouring matters as 

 obtained from plant, human, and animal sources. 

 Any possible criticism of the e.xpediencv of inserting 

 these chapters is somewhat disarmed at the outset by 

 Prof. Kayser's preface. He was himself somewhat 

 doubtful at first, but concluded that it would be in- 

 tolerable that a book on spectroscopy should pass 

 over these most important substances in silence. At 

 the same time, there is no doubt that much of the 

 work that has been published on these colouring 

 matters is worthless; indeed, these chapters serve to 

 show how chaotic and uncoordinated are the results of 

 investigators in these fields. 



The fourth chapter, which is from the pen of Dr. 

 .\. Pfluger, deals with dispersion, or, rather, anoma- 

 lous dispersion. Herein is to be found a fine critical 

 review of the experimental and theoretical work on 

 the relation between dispersion and absorption. 



The next two chapters, which are each about 270 

 pages long, deal with phosphorescence and fluor- 

 escence, the latter being contributed by Dr. H. Konen. 

 The division of the plienomena under the two heads 

 is based upon the criterion that phosphorescence per- 

 sists for a finite time after the e.xciting cause has been 

 removed. The chapter on phosphorescence, com- 

 mencing with an historical account of the subject, 

 deals in succession with the various means of excita- 

 tion and the experimental methods, the influence of 

 temperature, the spectroscopic investigation of phos- 

 phorescence, and finally the theories of the underlying 

 phenomena. This chapter contains a very full account 

 of all the work which has been carried out, and will, 

 indeed, prove very valuable. The literature is very 

 scattered, and this is the first time that a reasoned 

 attempt has been made to collect and correlate the 

 somewhat discordant details of experimental research 

 in the domain of phosphorescence. 



