coo SCIENCE PROGRESS 



through Hfe. The phenomena of taboo appear to possess the quaUty of an 

 " ambivalent " emotion — a strong desire to perform an action and a contrary 

 and more powerful impulse repressing that desire. The discussion of this 

 question in the second essay is very suggestive and enlightening. The other 

 chapters carry less conviction, and in particular some of the inferences in the 

 last chapter will probably strike most readers as very far-fetched. There 

 are a few printers' errors, e.g. " world " instead of " word " on page 112. 



A. G. T. 



ENGINEERING 



A Treatise on Airscrews. By Whyrell E. Park, A.R.C.Sc. (The 

 " D.U." Technical Series.) [Pp. xii + 308.] (London: Chapman & 

 Hall, 1920. Price 21s. net.) 



This publication is typical of the present somewhat breathless transition in 

 this country from regarding the mathematically equipped engineer as a freak, 

 or a joke, to a somewhat exaggerated notion of his possibilities. 



Mr. Park, as a Whitworth Scholar, received a training specially appro- 

 priate to an author in this series, inasmuch as his mathematical equipment 

 is for use and not for original work. His mathematical sections reproduce 

 in a satisfactory manner the better known variations of Drzewiecki's method, 

 and his notation and symbols are much better selected than usual, though 

 we still look in vain for any approach to the agreeable uniformity established 

 in German literature. 



But when he writes on the physical principles his of subject his training 

 leaves much to be desired : 



" , . , from the beginning design has assumed the nature of a compromise 

 between aerodynamic requirements and engine efficiency." 



"... the whole of design theory has been developed by mathematical 

 manipulation. . . ." 



" In the special case, when the airscrew is rotating and moving forward 

 in such a manner as to produce no disturbance of the air, and consequently 

 no thrust, the air behaves as a solid nut, and the analogy to bolt and nut 

 action is complete. . . ." 



" . . .a Newtonian Medium of small independent particles which is 

 mathematically continuous." 



The reviewer has marked thirty or forty passages such as the above, 

 some of which merely require rewriting to make them clear, but others of 

 which seem to show lack of clear view of the physics of the phenomena. 



The torque power coefficients in figures 13, 14 are spaced very unevenly, 

 whereas those in fig. 21 are spaced more evenly, w^hile the principles of inter- 

 polation would lead us to expect still more even distribution still. In fact, 

 the numerical illustrations give the impression of having been made up a 

 little artificially. The words " practical," " theoretical," " hypothesis," 

 etc., are used with considerable author's licence, sometimes almost in opposite 

 senses, in different places. 



Notwithstanding these limitations in the author's equipment, there is much 

 in the book which, if not original, is at least more accessible than elsewhere. 



He has the immense advantage of having an accumulation of test experience 

 at his disposal, and this may always be relied upon to keep the tedious com- 

 putations of actual design from any disastrous error. And the very appearance 

 of inconsistency is often a sign of freedom from slavery to an inadequate 

 hypothesis. 



To recount in detail the valuable aspects of the book is of course impossible, 

 but the student who has conscientiously worked through it will certainly be 

 equipped to take a place in the design room, though not perhaps in the 

 laboratory. 



The second part, embodying the author's ripe experience in giving shop 



