December 2 1, 1905J 



NA TURE 



>73 



LETTERS TO THE EDITOR. 

 [The Editor docs not hold himself responsible for opinions 

 expressed by Ins 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.] 



Radio-activity of Ordinary Matter in connection with 

 the Eartn's Internal Heat. 



Mr. Campbell's letter in your last issue (p. 152) re- 

 minds me of a point to which I have intended for some 

 time to direct attention. Prof. Rutherford (" Radio- 

 activity," second edition, p. 494) has calculated that the 

 radio-activity required to comjpensate for the earth's internal 

 heat is much exceeded by the (apparent) activity of ordinary 

 materials, as determined by me (Phil. Mag., June, 1903). 



Thus the smallest activity I observed was about io -4 

 times that of uranium nitrate, or 7X10-" times that of 

 radium : whereas the amount of activity, per unit mass 

 of the earth, required to compensate for the loss of internal 

 heat is only 4-6x10-", or less than one-thousandth part 

 of tin' activity computed to be actually there. 



We cannot well assume a much smaller apparent radio- 

 activity for the unknown material of the earth's interior, 

 for all materials hitherto examined have given effects of the 

 same order of magnitude, the radio-active elements, of 

 course, excepted. 



The simplest way out of the difficulty is to suppose that 

 the apparent radio-activity of ordinary materials is not a 

 volume effect of the same nature as that of radium, but 

 that it is merely a superficial effect of quite a different 

 kind, and only occurring at an exposed surface. It is 

 difficult to understand why Prof. Rutherford did not draw 

 some such conclusion. I do not know if he doubted the 

 correctness of my results. It is easy, however, to con- 

 firm them from other sources. Thus C. T. R. Wilson (Proc. 

 Roy. Soc, vol. lxviii. p. 158) found Ihe current through 

 ill'- in in a vessel of 163 c.c. capacity equal to 2-0x10-' 

 electrostatic units per second ; this is equivalent to 

 7X10-' 7 electromagnetic units. Madame Curie (" Thesis," 

 p. 14) found the current through a flat vessel of about the 

 same size, the bottom being covered with uranium nitrate, 

 equal to 7x10-" (electromagnetic). If this vessel had 

 been wholly lined with uranium, the current would have 

 been more than doubled, say 2Xio- 1: . Thus the ratio of 

 activities may be put at 3 X 10', as against 10' in my own 

 experiments. Considering the various activity of ordinary 

 materials, and the rough nature of the compa'rison in 

 both cases, this measure of agreement is fully confirmatory. 



As the question of the earth's infernal heat has been 

 raised above, I may mention that I am engaged on an 

 extensive investigation of the amount of radium contained 

 in various rocks. The majority of those rocks which I 

 have as yet tried (chiefly sedimentary) contain a good deal 

 more radium than the percentage which would keep up 

 the heat supply. Much more, however, must be done 

 before any confident statement can be made as to the 

 average amount of radium in the earth's crust. 



Experiments are also in progress with native iron, both 

 terrestrial and meteoric, with the idea that this may be 

 representative of the composition of the earth's interior. 



Sunnyside, Cambridge. R. J. Strutt. 



Magnetic Storms and Aurora. 



In his letter in your issue of December 14 (p. 152) Mr. 

 F. C. Dennett assigns a positiveness and a generality that 

 were not intended to the statement in my previous' letter 

 that on November 12 " no special auroral displav seems to 

 have been noted in this country." In vears of sun-spot 

 maximum, in the belt of greatest auroral" frequencv, nights 

 wholly free from aurora are probably the exception. In 

 the Shetlands, or even in the north of Ireland, aurora is 

 a much more common phenomenon than in the south of 

 England. 



On November 15 I learn from several sources that the 

 aurora was particularly brilliant, and the apparent move- 

 ments especially lively during the time 8.53 to 9.2S p.m., 

 the large declination movement occurred. Various 



wh 



Arctic explorers have stated that it is onlv when the 

 aurora displays this variable character that there is any 

 NO. l886, VOL. 73! 



clear connection between it and m; liances in 



those regions where both phenomena have their maxima. 

 1 li.' aurora on November 12 is said by the Lisburn observer 

 to have been of " the usual type " exhibiting " a steady 

 glow." It would be interesting to know whether this 

 aurora was observed at Lisburn or elsewhere during the 

 lime of the iai^e declination movement (6.30 to 7.10 p.m. 

 G.M.T.), and, if so, whether it then showed none of the 

 brillianf and variable phenomena seen during tin time 

 of the large declination movement three days later. 



For aurora to be observed on thirteen days in less than 

 three weeks must, I think, be rather an unusual event for 

 any place in England or Ireland ; the Lisburn observer 

 must keep a sharp look-out. Charles Chree. 



December 16. 



The Total Solar Eclipse of August 30. 



In visiting Spain at the end of August of this year I 

 was actuated by the desire once again, after an interval 

 of twenty-three years, to witness the marvellous and unique 

 phenomenon of a total solar eclipse. It is a sight which 

 cannot be imagined — it must be seen. Happening at a 

 time of maximum sun-spot frequency, it was reasonable to 

 expect a considerable displav of protuberances, and I 

 wished to form my own idea of their size by checking 

 their persistence or non-persistence through the phase of 

 mid-totality en a day which otherwise may be taken to 

 have been chosen at random. For this purpose a station 

 on, or very close to, the line of central eclipse was essential. 

 Torreblanca was chosen because it was the station of the 

 Barcelona and Valencia railway which was nearest to the 

 line of central eclipse, lying, in fact, about a mile to the 

 south-west of it. 



I observed the eclipse from the railway station, the 

 position of which is lat. 40 12' N. and long. 0° 12' E. 

 (Greenwich). The railway and official time in Spain is 

 that of Greenwich. By the clock at the railway station, 

 mid-totality occurred between ih. 18111. and ill. 19m. p.m. 

 Before the beginning of the eclipse I entered in my note- 

 book half the expected duration of totality, im. 50s. ; 

 when I had observed the second contact, I wrote the time 

 underneath, and, by addition, ascertained the time of 

 mid-totality by my watch. The display of protuberances 

 which appeared just before the moment of second contact, 

 and on the part of the sun's limb which was about to be 

 eclipsed, was, according to all witnesses, exceptionally 

 brilliant. When the time of mid-totality came round I 

 looked for these protuberances. They were absent. Not 

 a trace of them or of any others was visible to the naked 

 eye, and I searched the whole edge of the moon's disc with 

 the greatest attention. Their absence was confirmed by 

 Stephan (Comptes rendus, October 9), observing with the 

 best instrumental aid at Guelma. 



The sun's true altitude at Torreblanca on August 30, at 

 ih. 18.5m., may be taken as 54°-S- For this altitude the 

 augmentation of the moon's semi-diameter is i4"-3. Adding 

 this to the geocentric semi-diameter, 16' 21-4, as taken 

 from the Nautical Almanac, 16' 3S"-7 is obtained for the 

 apparent semi-diameter of the moon as seen from Torre- 

 blanca at mid-totality. Deducting from this the semi- 

 diameter of the sun, namely 15' 5o"-7, we obtain 45" as 

 a sufficient approximation to the width of the annular 

 band by which 'he disc of the moon overlapped that of 

 the sun. Therefore, to an observer stationed on the 

 central line in this neighbourhood, no protuberance could 

 be visible at mid-totality which had a height less than 

 45", and, neglecting the small displacement of Torreblanca 

 from the central line, the protuberances of second contact, 

 magnificent though they were, could not have exceeded this 

 height. 



Eight seconds before second contact I detected tin 

 streamers of the outer corona on the western limb of the 

 moon. At this moment there was no trace of the inner 

 corona, which presents to the spectator during the whole 

 of totality the appearance of a bright, luminous ring 

 surrounding the moon. 



If we assume that the argument from parallax is applic- 

 able to the inner corona, as it is to the protuberances, we 

 have to conclude that, eight seconds before second contact, 

 the light-giving portion of it did not extend further than 



