March 22, 19 17] 



NATURE 



duty of cultivating- it in the best possible manner ; 

 on the urban side the people must realise that the 

 country ought never ag^ain to be so dependent on 

 sea-borne food as it has been during the past fifty 

 years. Henceforth, Mr. Turnor urges, security of 

 supply must be the motto, instead of a cheap 

 supply at all hazards ; and, lastly, the workers 

 themselves must have a new outlook, and realise 

 that salvation for our future economic troubles 

 lies in unrestricted, and not in restricted, individual 

 -output. 



Mr. Turnor argues his case extremely well, and 

 ■drives home his arguments with numerous dia- 

 grams illustrative of his statistics. 



One of the secondary effects of the war is 

 that agriculture is fast becoming a controlled in- 

 dustry, and experiments in organisation are being 

 tried now on a vastly larger scale than before. 

 Already some of the sup-p-estions of the reformers 

 have been carried out. We have minimum prices ; 

 ■we shall soon have a minimum wage. The Game 

 Laws have had a hole knocked through them, and 

 in several directions the new^ conditions are 

 advanced beyond the wildest dreams of 1914. We 

 shall soon "see how the new order is going to work, 

 and in the meantime we can only welcome the 

 fullest discussion of the agricultural problem as 

 it is and as it seems likely to shape itself. 



Morphology of Invertebrate Types. By Dr. A. 



Petrunkevitch. Pp. xiii + 263. (New York : 



The Macmillan Co. ; London : Macmillan and 



Co., Ltd., 1916.) Price 6s. net. 

 This volume is divided into twenty-eight chapters, 

 •each dealing with one type. Each chapter con- 

 sists of a description of the systems of organs of 

 a type and a series of instructions to be followed 

 In the examination of the animal. Much import- 

 ance is rightly attached to the drawings which 

 the student is directed to make, and to do away 

 Avith any tendency to copy the figures included in 

 the book these are either diagrammatic or repre- 

 sent some related type. This latter device has 

 liere and there its disadvantages, e.g. the figures 

 of Ankylostoma and Wilsonema are of relatively 

 little use in aiding the student to understand the 

 structure of Ascaris; there are too many differ- 

 •ences between Ascaris and the types figured. 



The descriptions are accurate, and on the whole 

 well done, though some parts are too short, e.g. 

 the accounts of the nephridia of the earthworm 

 and of Nereis are inadequate. The types chosen 

 are all found in America, except the Trematode 

 Dicrocoelium lanceatum and the medicinal leech, 

 iDut many of them occur also in Britain or are 

 •closelv similar to British species, and the book 

 will therefore be helpful to those " on this side " 

 who desire an account of the general anatomy of 

 such tvpes as Pennaria, Sertularia, Tima, Aurelia, 

 Dendrocoelum, Daphnia, a spider (Agelena), 

 Asterias, Venus, Limax, Loligo, and Molgula. 

 But in most laboratories in this country- where 

 similar types are studied, probably more attention 

 is devoted to the finer structure of some of the 

 organs, e.g. the nephridia cited above. 

 NO. 2473, VOL. 99] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Horizontal Temperature Gradient and the Increase 

 of Wind with Height. 



If the horizontal layers of air were isothermal (the 

 upper layers having the lower temperature), then the 

 gradient winds at different heights would be propor- 

 tional to the temperatures (absolute) at those heights. 

 Consequently the wind would decrease with height, 

 and although a higher temperature at a given altitude 

 over the higher pressure is a necessary coroUarj' of an 

 increase of wind with height, the converse is not 

 necessarily true. It is clear on reflection that with 

 such a temperature distribution as that described 

 above, the temperature at any point in BD of Mr. 

 Dines 's diagram in Nature of March 8 (p. 24) would 

 be below the temperature at the corresponding point 

 in AC, so that BD would be less than AC, and conse- 

 quently V less than V ; but the actual relation runs 

 s6me risk of being obscured by Mr. Dines 's use of iso- 

 baric surfaces which in other respects gives an admir- 

 ably simple exposition of a theorem in atmospheric 

 dynamics, and shows also that if the isobaric and 

 isothermal surfaces coincide there is no variation of 

 wind with height. Incidentally, during the past winter 

 months the mean isotherms have run from N.W. to 

 S.E., and have given at 6000 ft. a N.W. "thermal " 

 wind of about 1^ metres per second superposed on the 

 wind between 1000 and 1500 ft. E. Gold. 



Meteorological Section, R.E., March 16. 



A Fixed System of Grating Interference Bands. 



I WISH to direct attention to a very remarkable 

 property possessed by one of the systems of inter- 

 ference bands which make their appearance when 

 white light is reflected from a plane replica-film 

 grating, backed by a parallel silvered surface 

 separated by a film of air. The appearance of the 

 fringes in question is shown in the accompanying 

 photograph, when the grating- has 14,508 lines to 

 the inch and the air-space is 00740 cm. 



This system of bands, when examined by a spec- 

 trometer with a fixed collimator, remained absolutely 

 fixed in the field of view of the telescope as the 

 grating was rotated. This rotation has the effect 

 only of moving the spectrum to and fro across the 

 field of view without, in the least, altering the posi- 

 tion of the dark fringes. This important property, 

 which is uniquely exhibited by these fringes, seems 

 to have escaped the notice of Prof. C. Barus, who 

 has dealt with grating interferences and with their 

 application to the displacement interferometry in a 

 series of papers which appear in a collected form in 

 the monograph "On the Production of Elliptic Inter- 

 ferences in Relation to Interferometry" (Carnegie 

 Publication No. 149, 1911). 



