November 19, 1908] 



NA TURE 



67 



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. 



An Electromagnetic Problem. 



In the application of general principles to special cases 

 it is sometimes found that the result is a seeming paradox, 

 which is not always easy to remove. Such problems, 

 although involving no new principle, are nevertheless of 

 considerable interest, and after attaining their satisfactory 

 solution we often realise that we did not before appreciate 

 the full import of the general law. 



The following question has been discussed with con- 

 siderable interest among some of the writer's friends, and 

 therefore it seemed not improbable that other physicists 

 might be interested. 



If two spheres of positive electricity are near together 

 and are suddenly released, it is clear that their potential 

 energy decreases as they separate and goes over into 

 kinetic energy of motion. This kinetic energy is, of 

 course, the energy of the magnetic field which results from 

 the motion of the charges. 



It seems possible, however, to arrange a system so that 

 tills magnetic field shall vanish because of symmetry, and 

 the question then presents itself. Where is the energy? 

 Suppose we have a sphere of positive electrification placed 

 as the water is in a soap bubble, and capable of expand- 

 ing under the mutual repulsion of its parts. The potential 

 energy of the electricity certainly decreases as the sphere 

 expands, and if the electricity be considered continnotis 

 there is certainly no chance for a magnetic field, as is 

 easily seen from consideration of symmetry. If the sphere 

 be allowed to expand, where does the energy go? The 

 obvious answer is that the electricity is not continuous, but 

 exists as discrete particles, i.e. as electrons ; but if we 

 try to escape the difficulty in this way, it is equivalent 

 to admitting that the electrical laws, together with the 

 conservation of energy, require in themselves the discrete 

 structure of electricity. If, on the other hand, we say 

 that the electricity is associated with matter, i.e. with 

 ponderable mass, and the energy appears as ordinary 

 mechanical energy of motion, then we are admitting that 

 the electrical and energy laws require the association of 

 electricity with matter. 



There seem to be no other solutions to the problem than 

 those above given, and if we admit either of them we 

 reach a conclusion which certainly is striking when we 

 consider that we have only used the general laws of elec- 

 tricity and energy. 



'I he writer does not state tlie above as a fundamental 

 [lar.idox, but only as an interesting problem. 



D. I". Co.M STOCK. 



Institute of Technology, Boston, Mass., November 3. 



The Progress of Aviation. 



I HAVE read, with great interest the article on the above 

 subject by Prof. Bryan in Nature of October 29. 



May I be permitted to direct especial attention to thi- 

 necessity for finding the displacement of the centre of 

 pressure on all kinds of surfaces and at all angles therein 

 referred to? The paper by Prof. Bryan and Mr. Williams 

 on the subject of longitudinal stability, and Captain 

 Ferber"s article in the Rcviic d' Arlillerie (November, 1905), 

 both assume the truth of Joessel's law. There is, how- 

 ever, evi-ry reason to suppose that there is a certain critical 

 •■"c,'lr- below which Joessel's law ceases to be true, the 

 .1 icement decreasing with the angle instead of in- 

 i~ing.' 



Consequently, the numerical conclusions arrived at from 

 the stability formulse of Captain Ferber and Prof. Brvan 

 may be very wide of the mark. 



' SorailMoedebeck's "Pocket-book of Aeronautics" (igoi) : Wilbur 

 Wrieht, Smirhsonian Report, 1012, np. i33-'48 t Journal 0/ IVesUrn Society 

 vflingincers, December, igoi) ; Turnbull, Physical Review, vol. xxiv.. 



NO. 2038, VOL. 79] 



I hope to experiment in this direction myself, but my 

 time is very limited. There can be no doubt whatever 

 that a thorough investigation as to the centre of pressure 

 would be of the greatest practical use. 



Herbert Chatley. 



32 Britannia Road, Southsea, October 31. 



I .^GREE strongly with all that Mr. Chatley has said. 

 It cannot be too emphatically pointed out that the object 

 of our stability investigations was to show that the subject 

 is capable of being treated mathematically, and that, given 

 the requisite experimental data, the conditions of stability 

 of any system of planes or surfaces can be calculated out 

 in the form of numerical results. The cases in which this 

 was done were intended merely as examples illustrative of 

 the general method, and for this purpose Joiissel's law 

 furnished the simplest assumption available at the time. 

 It will be noticed, too, that arbitrarv values were assumed 

 for the moments of inertia of the systems. To draw infer- 

 ences from the results of examples worked out with this 

 object would be an unfortunate mistake. 



It is to be regretted that want of time has prevented 

 my attempting to work out any examples based on the 

 Turnbull results, though the idea suggested itself when I 

 saw the paper in the Physical Review. The theory of 

 stability has thus been somewhat at a standstill. Those 

 who, like Mr. Chatley and myself, would like to see that 

 theory advanced are prevented from doing this by pressure 

 of other duties, while those who have the necessary time 

 and money have been mainly occupied of late in breaking 

 records. Mr. Lanchester's theory of stability starts from 

 so different a standpoint that it must be discussed at a 

 future time. G. H. Bryan. 



Potato Black Scab. 



The discovery this autumn of blacl-c scab in the potato 

 crop in two localities in co. Down was the means, through 

 the Irish Department of Agriculture, of supplying me witli 

 excellent material of diseased tubers for examination. I 

 have kept the resting " spores " of the chytridian fungus 

 Chrysophlyctis endobiotica, Schilb., causing the disease, 

 under varied conditions of temperature, nourishment, 

 moisture, and light, and have succeeded in causing the 



spores " to germinate, especially by cultivation in potato 

 juice. Each " spore " proves to be a zoosporangium, full 

 of zoospores or zoogonidia, seen in active swarming 

 motion before rupture of the sporangium. The zoospores, 

 I '5-2 fi in diameter, escape through a slit-like opening 

 in the wall of the sporangium 30-60 /i in diameter, and 

 have the usual characters of a chytridian zoospore. 



Since the publication of Schilbersky's short preliminary 

 account in 1896 in the Berichte der deutscher botanischen 

 Gesellschaft, and Potter's account of his discovery of the 

 pest in Cheshire in 1902, we have learnt nothing of the 

 life-history of this injurious fungus. 



T. Johnson. 



Royal College of Science, Dublin, November 17. 



The Nature of 

 ExfERiMEXTS bv Prof. Bragg 



Rays. 



and myself upon the 

 secondary kathode radiation which proceeds from matter 

 through which 7 rays are allowed to pass, taken in con- 

 junction with the similar result announced by Mr. Cooksey 

 in Nature of April 2 (vol. Ixxvii., p. 509) for X-rays, 

 support the theory of the material nature of X and of 

 y rays originally advanced by Prof. Bragg. 



The modification of the ether-pulse theory recently 

 advanced by Prof. Thomson may possibly furnish a partial 

 explanation of these effects, but in the light of some 

 experiments which I have lately carried out upon the 

 secondary y rays, even this modification seems quite in- 

 sufficient. .A brief summary of these results is appended. 



(i) The y rays of Ra, and probably of Th, appear to 

 consist of two distinct homogeneous bundles, the value of 

 A/A (where \ is the absorption coefficient and A the 



