436 



NATURE 



[January i, 1920 



therefore, be other kinds of mim to harbour the 

 larvae of },lusca, domestica through the winter. 

 Monreith. Herhekt Maxwkll. 



The Magnetic Storm of August 11-12, 1919. 



The Kodaikanal Observatory magnetographs re- 

 corded tlie "sudden commencement" of this storm 

 on August II at 6h. 58m. G.M.T., horizontal force 

 showing an instantaneous rise of 1497 and vertical 

 force about 357, while the declination magnet was 

 deflected about i' towards west. 1 have measured the 

 three traces, and, after making due allowance for the 

 errors affecting the hour-marks, the following values 

 were obtained : — 



h. m. 



Horizontal force 6 580 



\ertical force 6 587 



Declination 6 579 



Greater weight may be given to the declination _ result 

 because of the more sharply defined hour-marks in this 

 trace. 



It is of interest to compare this result with the times 

 recorded in England in view of the fact that at 

 Kodaikanal the sun at 6h. 58m. G.M.T. was barely 

 three minutes past meridian passage and only 5° north 

 of the zenith. In other words, this observatory was 

 almost at the centre of the earth's disc as seen from 

 the sun, and it might be supposed that the disturbance 

 would have been recorded here earlier than at other 

 places if it is directly due to emanations from the sun. 

 .•\pparentl\- the sudden commencement was recorded 

 at Kew '-at about yh. G.M.T.," and at Stonyhurst at 

 6h. 50m., while at Eskdalemuir the time is given 

 definitely as bh. 58m., in exact agreement with my 

 result (flATURE, vol. ciii., pp. 483, 505 and 506). 



It seems probable that the impulse is simultaneous 

 over the earth to within a fraction of a minute, but it 

 would be interesting to know the limits of error to 

 which the above times are subject, and particularly 

 whether allowance has been made for the rather large 

 error which may be produced by the mechanism for 

 cutting off the light from the sensitive paper at each 



hour. J. EVERSHED. 



Kodaikanal, S. India, November 25. 



Deflection of Light during a Solar Eclipse. 



During a total eclipse of the sun there will be, as 

 I suppose, an increase of density of the air at the 

 central portion of the shadow. If we imagine the 

 normal atmosphere removed, we are left with a 

 residual atmosphere the refraction effects of which 

 will be changes in the normal refraction effects. The 

 whole point is whether, on reasonable suppositions, 

 this residual atmosphere can produce refraction effects 

 of the order of the observed effects that have been 

 attributed to the gravitational field of the sun. 



Let us suppose with Prof. Eddington that the por- 

 tion of this residual atmosphere concerned correspond- 

 ing to a star near the edge of the sun's disc has a 

 radius of about 150 yards, and let the index of refrac- 

 tion of the residual atmosphere at the central part be 

 (U,, and that at the circumference fi,. Then, as I 

 have shown, the displacement will be accounted for if 



(-,i) 



^2 = Ml 



Assuming that the change of density depends onlv 

 on change of temperature, we have 



Pi ^1 "i-i' 

 where p,, p,, 0,, 6^ are the increases of density and 

 falls of temperature at the centre and circumference. 



NO. 2618, VOL. 104] 



Thus we have, approximately, 



^2zli= __7 . ^2-^1- 7 

 e, 10V3-1)' <*! io'(f'i-') 



We must now make some assumption as to the 

 gradient of temperature, and this is a very important 

 {Xjint. It would not, I think, be right to malce it 

 uniform or approximately uniform throughout a dis- 

 tance of 150 miles. We will assume that in a dis- 

 tance of 150 vards the increase of temperature is 

 I /100th part of Sj. This does not seem to be un- 

 reasonable. 



We then have /i.,= 1000007 for 'he index of refrac- 

 tion of the residual atmosphere at the centre, corre- 

 sponding to a lowering of temperature of 628° C. at 

 the centre and of 622° C. at a distance of 150 yards. 

 If we assume that the increase of temperature at a 

 distance of 150 yards is l/ioth of 6,, we get the fall 

 at the centre 063° C. and that at 150 yards' distance 



a d 



os7° C. It will be seen that the greater —^-^< the 



smaller is the necessary fall of temperature at the 

 centre. .Vlexr. .Anderson. 



I'niversitv College, Galway, December 28. 



Entente Scientific Literature in Central Europe during 

 the War. 



I WAS much interested in reading Prof. Brauner's 

 letter from Prague in Natlke of December 11. Like 

 Prof. Brauner, I was unable to obtain Nature during 

 the first two years of the war, and I fully appreciate 

 his joy on obtaining your invaluable journal again, 

 after an interval of more than five years. 



Prof. Brauner states that from July 30, 1914, "the 

 .\ustrian Government prohibited for more than four 

 vears the circulation of anything printed in England 

 as a punishment for the regard which, especially 

 during the war, we [the Czechs] have always had for 

 your country." 



This statement is misleading, for "enemy" 

 periodicals were withheld from the whole of the 

 .\ustrian Empire, and not from the Bohemians alone. 

 Prof. Brauner is apparently unaware of the fact that, 

 from 1916 onwards, it was possible for institutes of 

 the .Austrian universities and technical high schools 

 to obtain scientific periodicals and publications from 

 Entente countries. The enactment which made this 

 Dossible did not appear to be generally known, but 

 I am aware of two institutions at least, which made 

 aoplication for, and obtained from the .Austrian 

 Foreign Office, the necessary permission ; and in 

 neither instance was any difficulty experienced. 



The Radium Institute of Vienna was one of the 

 institutions concerned, although at that time several 

 Poles (one of thein a Russian subject) and the writer 

 of this letter were working there. What is more, the 

 -Austrian Foreign Office was not unaware of this 

 "quasi-international" character of the Radium 

 Institute ! 



Books were also obtainable, and I know of several 

 men of science of Vienna, Budapest, and even of 

 Prague, who were granted permission and obtained 

 books from France and England through neutral 

 countries. 



A few months before the armistice I remember 

 sitting in a A'iennese restaurant at the same tabic as 

 a gentleman, who was voraciously devouring the con- 

 tents of the Sketch and the Illustraled London News. 

 To judge from his frequent unsuppressed laughter, 

 one would have thought he was scanning the pages 

 of Punch. Not having seen these periodicals for 

 nearly fiv6 years, my interest and curiosity were 

 aroused, and I asked this gentleman's permission to 



I 



