jo-v 



NAT URL 



[Decembi B hi. 1918 



explanatory matter has been inserted occasionally, 

 and the examples have been much improved bj 

 the inclusion of hints for their solution. In one 

 or two places the clearness of the original is some- 

 uh.it marred, as, e.g., on p. 91, where two dif- 

 ferent notations are used simultaneously. It is 

 not always obvious why formulae have been ban- 

 ished from the text to separate lines, and vice 

 versa ; but this is a question of taste. The plates 

 have been touched up here and there, and occa- 

 sionally corrections have Keen mack-, e.g. is., .;, 

 5 ; \i\ ., 10, [8. 



It is surprising thai the editor has retained the 

 definition of curvature as measured by the diameter 

 of curvature, whilst in at least one place he has 

 used the first person in an interpolated remark. 



The index and the classified list of curves 



are welcome additions. S. Brodetsky. 



Ik lecount of the Crustacea of Norway. With 



short descriptions and figures of all the Species. 

 By Trot'. G. O. Sars. Vol. vi., "Copepoda, 

 Cyclopoida." Parts 9—14. 1'p. 105 225 + 54 

 plates. (Bergen: Published by the Bergen 



Museum. ic)i 5 [S. ) 



Willi tin issue of the six parts noted above, 

 Prof. Sars concludes the sixth volume of his great 

 work on the Crustacea of Norway, the third of 

 the series to be devoted to the rich and varied 

 group of the Copepoda. This volume deals with 

 the division Cyclopoida, which includes, for the 

 most part, bottom-haunting species, many of 

 them parasitic or semi-parasitic. Like the Har- 

 pacticoida treated of in the preceding volume, 

 they are rarely found in the plankton, and must 

 be sought for by special methods of collecting 

 demanding much skill and patience. For this 

 reason they have received far less attention than 

 the relatively few species that are captured in 

 bulk by the lou-net, and the proportion of novel- 

 ties described in these volumes is very high. It is 

 scarcely too much to say that Prof. Sars is giving 

 us, for the first time, the means of forming .1 

 just impression of the Copepoda as a whole, both 

 as regards their structural diversity and their 

 habitats and distribution in northern seas. The 

 species living in fresh water were previousl) 

 better known, but here also Prof. Sars's accurate 

 drawings will greatly facilitate the identification 

 of species. 



Apart from the faunistic value of the work, 

 however, tin- iconography which it provides for 

 many groups hitherto sadly in want of illustra- 

 tion will be oi the greatest value to the morpho- 

 logist and taxonomist. Among the numerous 

 points oi interesl touched on in the parts under 

 review, it may be noted that Prof. Sars revives 

 Thorell's group of the Poecilostoma, although in 



a restricted sense- and a subordinate position. In 

 doing so he discards Claus's interpretation of the 

 mouth-parts and reverts to that originally give' 1 

 by Thorell, according to which the mandibles, 

 elsewhere so persistent, have entirely disappeared 

 in the species composing this aberrant group. 



W. T. C. 

 NO. ^564, VOL. I02] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can lie undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Naturf.. No notice is 

 taken of anonymous communications.] 



The Perception of Sound. 



I regrei thai I overlooked Prof. Bayliss's letter in 

 Naturi "I October 17, in which he made an appeal 

 for ui\ opinion. Hut, if 1 right!} understand, the 

 question at issue seems to hi- mainh one oi u 

 Can we properh, speak oi the propagation oi si 

 through an incompressible fluid? I should ans 

 Yes. ["here max he periodic motion and periodic 

 variation of pressure; the fact that there an- no varia- 

 tions of density seems immaterial. Consider plam 

 waves, corresponding with a pure tone, travelling 

 through aii. In every thin layer of air and thin 



means thin relath eh to the wave-length - in 



periodic motion and periodic compression, app 

 n: ! uniform throughout the layer. Hut tin 1 

 pression is nol essential to die travelling of tin- sound. 

 'The- substitution of an incompressible fluid of the- same 

 densit\ 1,11 the gas within the layer would l» no 

 hindrance-. Although there- is no compression, there 

 remain a periodic pressure and a periodic motion, and 

 these suffice to carry on tin- sound. 



The case is even simpler if we are prepared to 

 contemplate an incompressible fluid without mass, for 

 then the layer need not 'be thin. The interposition of 

 such a layer has absolutely no effect, tin- motion and 

 pressure at the further side- being the s;une- as if the 

 thickness of tin- layer were reduced to zero. To all 

 intents and purposes the sound is propagated through 

 tin- layer, though perhaps exception might he- taken to 

 the use of the- word propagation. 



\s legends tin- ear, we have to consider the 

 behaviour of water. From some points of view the 

 difference between air and water is much more "in 

 of density than of compressibility. The velocities o) 

 propagation are only as 4 or 5 to 1, while- the 

 densities arc- as 800 to 1. Within the cavities of the 

 ear, which are small in comparison with tin 

 lengths of musical sounds, the water iti.ee certainly 

 he- treated as incompressible; but the fact does not 

 see in 10 In- of fundamental importance in theories of 

 audition. Is N 1 i.ie.n. 



Terling Place, William. 



The Common Cause of Pure and Applied Science. 



\ c.eeeii) deal of anxiet\ has been expressed recentl) 

 in various quarters lc-st tin- great interesl now heinL; 



evinced in applied science men perpetuate-, 01 even 



a i ' .en-, tin- national neglect of pure science. As 

 I do nol share- this anxiety, but, on the- contrary, am 

 Strongl) convinced that exactly the opposite effect will 



ensue; and as there- seems to be some danger in the 

 attitude- thai is being assumed In Certain of m\ 

 friends, I should like', with greal respect, to ask for 



attention to the- considerations which have led me to 

 these opinions. 



For many Mens pasl there lias been in this country 

 what Mi. late Lord Armstrong once- well called " ■> 

 vagui en for technical education," a dim feeling on 

 the- pari of the industrial world, collectively speaking 

 (there have, oi course, been brilliant individual excep- 

 tions), that there was somi business-end oi sciehci 



then was worth gelling hold of, and that should be 

 gOl on '-.i-c te-rnis of talent, time, and money. \V< 

 know what ii has laded to produce in institutions, irt 

 individuals, and in industrial efficiency ; and we knew 



