NATURE 



[January 5, 1922 



septic and innocent quality of the sunlight just as we 

 get it after filtration by the '"unpolluted atmosphere." 



But after seeing the clinical action of sunlight at 

 Leysin under Dr. Rollier and at the Treloar Hospital 

 under Sir Henry Gauvain, and reading the papers 

 of Sonne (^Acta Medico, Scandinavica, vol. 54, fasc. 4, 

 •'The Mode of Action of the Universal Light Bath," 

 from the Laboratory of the Finsen Medical Light 

 Institute, Copenhagen) mentioned in my previous 

 letter, I am absolutely certain, as anyone else 

 would be, that there is more in the curative action of 

 sunlight than its bactericidal effect. In a recent 

 lecture before the Physiological Society of University 

 College, entitled "The Physiology and Therapeutics 

 of Sunlight: Facts and Questions/' I cited instances 

 and showed photographs of many cases where the 

 value of sunlight could not have depended upon its 

 antiseptic power. Sonne's view is that sunlight 

 warms the blood without appreciably raising the 

 general body-temperature, that thi-s produces the 

 valuable, without the injurious, effects of fever, 

 and that this action is obtainable by the proper use 

 of sunlight, and by that alone. 



The practical importance of this fascinating physio- 

 logical problem is apparent to me after recent visits 

 to certain sanatoria, otherwise admirable, where I 

 have been told that I should see the sunlight em- 

 ployed, and have found, for instance, that open air 

 and diffused daylight, the latter reaching the face and 

 possibly the hands, were regarded as the equivalent 

 of Rollier 's treatment ; or that the children were 

 scrupulously put under awnings or sent to school in an 

 adjacent wood whenever the sun shone. The pitiful 

 statistics of these places, compared with those of 

 Rollier and Gauvain, point the moral. 



Since first drafting this letter I have seen, thanks to 

 Prof. Leonard Hill, new records of work done by 

 Prof. A. F. Hess in New York, showing the cure of 

 experimental rickets in animals fed and continuing to 

 be fed on a diet which invariably produces rickets — 

 when they were placed in sunlight for a few hours 

 daily. No mere antiseptic action is here in question. 



\yith rare exceptions, we do not yet know what 

 heliotherapy consists of in this country ; no one yet 

 knows its action, nor even the pure physiology ' of 

 sunlight. Meanwhile, we are carefully depriving 

 many patients of their one chance of life, and quacks 

 and others are using all manner of artificial lights 

 in therapeutics as if they were equivalent to, or 

 better than, the sunlight, which, according to Sonne's 

 experiments — nicely consorting from another point of 

 view with those of Sir Oliver Lodge — is incomparable. 

 _ The Smoke _ Abatement Committee has now pub- 

 lished its admirable Final Report, and reiterates now 

 my plea for an inquiry such as Carrel, I am told, is 

 about to undertake at the Rockefeller Institute in New 

 York, but which no one, not even Prof. Leonard Hill, 

 our great student of the air and temperature relations 

 of the body, is yet making here into the action of sun- 

 light. I believe that the restoration of sunlight to our 

 urban populations, mostly darkened in slums and 

 smoke, is the next great task of hygiene in this 

 country. _ C. W. Saleebv. 



Royal Institution, January 2. 



Units in Aeronautics. 



A LARGE number of equations in aerodynamics 

 appear in the form R = fepSV\ where R, p, S, V are 

 the reaction, density of the atmosphere, surface, and 

 relative wind velocity. 



Since both R and pSV have the dimensions of 

 force, k is clearly a numerical coefficient unchanged 

 NO. 2723, VOL. 109] 



by transformation from one self-consistent system of 

 dynamical units to another, e.g. from foot, pound, 

 second, poundal to centimetre, gram, second, dyne. 



In transforming to an inconsistent system, k will 

 in general be altered, but the inconsistencies may 

 cancel each other in particular cases so far as to leave 

 k unaltered. For example, if a gravitational unit of 

 force is introduced g times the consistent unit, then 

 we may write R = fepSV7^ pounds weight in the 

 British system, or grams weight in the cg.s. 

 system. And while the numerical values of both R 

 and g vary inversely as each other, the value of k 

 is not affected if the system is otherwise consistent. 



Prof. Bairstow ("Applied Aerodynamics," p. 119) 

 maintains that the gravitational unit of force is the 

 natural one, and to get rid of the local value of g 

 that mars the consistency of his dynamical equations 

 he introduces a new unit of mass, the "slug" of 

 (local) g pounds mass, the use of which he restricts to 

 the measurement of atmospheric density. Then putting 

 p'—pIS he can write R = fep'SV- in the same form 

 externally as before, and read off the value of R in 

 pounds weight instead of in poundals, all without 

 showing g explicitly. 



In this way, in his opinion, "it appears that the 

 divergence of language {sic) between science and 

 engineering would disappear." 



But if_ we apply this method of measuring the 

 density of the atmosphere to the estimation of the 

 lift of an aerostat we get, putting m=density of 

 hydrogen /density of air, (i-m) p'xvolume = R slugs 

 weight {=gR pounds weight^^^R poundals!). 



Altogether it seeme more satisfactory to teach 

 engineers the physical meaning of Newton's laws of 

 rriotion than to invent units which evade them, just as 

 it has proved better to teach them the elementary nota- 

 tion of the infinitesimal calculus than to devise 

 " calculus-dodging " demonstrations. .'\. R. Low. 



London, December 8. 



Self-fertilisation in Mollusca. 



As the question of self-fertilisation in the more 

 highly organised invertebrates is of considerable im- 

 portance from the genetic point of view, I would like 

 to direct the attention of readers of Nature to a pub- 

 lication (Acta Soc. pro Fauna et Flora Fennica, vol. 40, 

 No. 2, 1915), a copy of which I have just received 

 from the author. Dr. A. Luther. It constitutes an 

 important addition t© the evidence for the occurrence 

 of self-fertilisation in mollusca, as Dr. Luther states 

 that he succeeded in rearing two generations by self- 

 fertilisation in Agriolimax agrestis. I have recently 

 pointed out (Proc. Malac. Soc., vol. 14, 192 1) the 

 value of Kiinkel's work on Arion in this respect; 

 but at the time Dr. Luther's results were not known 

 to me, as the publication was not available and had 

 not figured in the "Zoological Record." Should 

 these important observations be finally confirmed and 

 the technique improved so as to produce more than 

 two generations, a very valuable contribution to 

 genetic study will be achieved. Dr. Luther's work, 

 however, emphasises the necessity for conducting a 

 study of environmental conditions in order to secure 

 improved viability. 



I may point out, perhaps, that though quite a 

 number of cases of self-fertilisation have been recorded 

 in Pulmonata by Lang, Holzfuss, and others, the 

 subject is by no means fully explored either with 

 regard to the distribution of the phenomenon among 

 Pulmonata or the circumstances in which it occurs. 



G. C. ROBSON. 



British Museum (Natural History), 



Cromwell Road, London, S.W.7, December 23. 



