2 So 



NA TURE 



[January 7, 1909 



equation siibslitutc 2Va, where a is the area of the spot 

 measured in terms of the solar hemisphere. This leads to 

 B = 3Fa. 

 The vertical force will not be constant over the whole 

 spot area, but wo may now take F to be the average 

 vertical force, and its variations are immaterial when it 

 is only a question of evaluating the order of magnitude of 

 the effect. At the distance of the earth the corresponding 

 force acting in a direction radial from the sun is equal to 

 •^Faa^, where a is the angular semi-diameter of the sun 

 as viewed from the earth (a'=io-'). 



If we adopt Zeeman's estimate of 3000 C.G.S. for F, 

 the onlv remaining cjuantily to be estimated is a. We 

 niav take account of the greatest possible collective effect 



of all spots by imagining ihem all of 



the same polarity and placed at the . 



centre of the solar disc; a then re- 

 presents the total spotted area which 



may be obtained by reference to the 



Greenwich tables. If we include in 



the spot area the penumbra as well 



as the umbra, we find the averai^e 



value to be about 500 in millionths 



of the sun's hemisphere, but in years 



of sun-spot maxima this number will 



be considerably exceeded. Taking the 



year 1893, which w^as the most 



prominent sun-spot year since 1870, 



I find that on August 7 the value of 



II was 5128 in the above units. 

 Writing 0005 for a, the radial solar 

 force is found from the above ex- 

 pression to be 4.5x10-° C.(i..S. On 

 the Greenwich curves such a force 



would be represented by a displacement of one-n-mh ol a 

 millimetre. 



The magnetic fields on the sun, on which our calcula- 

 tions are based, cannot, of course, form or disappear 

 suddenly, and in view of the smallness of the effect the 

 only question that can arise is whether, if persistent for 

 a sufficiently long time, periodic variations might iust be 

 traceable. Some years ago 1 worked out in full the 

 periodic effects of a magnetised and rotating sun (Phil. 

 Mag., vol. xlvi., p. 395, 1898). The main periods intro- 

 duced are two, one of twenty-nine days, if the synodic 

 revolution of the sun is twcnty-.seven days, and one equal 

 in length to the sidereal day. (The period of twenty-seven 

 days drops out altogether.) As regards the former period, 

 it could only show itself if the magnetic state of the sun 

 persisted for a sufficiently long period, and would probably 

 even then escape detection owing to its smallness. 



Similarly, the sidereal period would be hidden behind 

 that of equal period and much larger amplitude which 

 is introduced bv the seasonal fluctuation of the ordinary 

 diurnal period ; but we need not enter into a detailed dis- 

 cussion of the possibility of tracing minute effects. My 

 main object has been to show that the magnetic disturb- 

 ance hitherto associated w-ith the state of the sun's surface 

 ■ annot be accounted for by the direct magnetic action of 

 Prof. Hale's electric vortices. 



.'XrTIII-R ScMfSTER. 



Kew Records of the Italian Earthquake. 



The accompanying traces show records of the Ital'an 

 earthquake of December 28, 190S, given by the Milne 

 seismograph and the declination magnetograph respectively 

 at Kew. The former shows a very sudden arrival of 

 seismic waves of some magnitude at 4h. 23.6;n. a.m. 

 (G.M.T.). If these represent " preliminary treinors " 

 from an earthquake originating in Italy, about 2000 kilo- 

 metres away, one would expect the large waves not to 

 commence until some seven minutes later. Ordinarily, the 

 maximum amplitude occurs some little time after the large 

 waves commence. In the present case, however, the 

 maximum is shown at either 4h. 3i-im. or 4h. 32-7m., 

 the lateral traces overlapping at. both these times. This 

 means a movement of more than 17 mm. at the end of 

 the boom. 



It is unusual lor the Kew magnetic curves to show 

 any record whatever of earthquakes. On the few past 

 occasions when the magnetic traces have shown anything, 

 it has taken the shape of a burr of vague outline, indicating 

 that the magnets have been put in oscillation. (Ordinary 

 magnetic changes are not discontinuous, and for these the 

 magnets are practically dead-beat.) The present occasion 

 is, I think, unique in that not one, but several, of these 

 burrs are distinctly visible in the original declination curve, 

 though in a copy they will, I fear, be indistinct. The 

 time scale is only 15mm. to the hour, and an uncertainty 

 of at least 0-5 minute must be allowed in estimates of time. 

 To prevent prejudice, the four burrs distinctly seen were 

 measured prior to an inspection of the Milne trace, and 

 the times allotted were respectively 32, 30, .;o, and 47 



■♦•M"*-*'? .few U^J^^Ovrp 





J 



minutes after 4 a.m. These answer apparently to thi/ 

 seismic movements of which the measured limes are 32-7 

 (or 31-1?), 355, 39-6, and 467 minutes after the hour. 

 The original declination trace shows a distinct movement 

 at 4.27, but this is of a different type, and may not be of 

 seismic origin. There is alsso at 4.23 or 4.24 a very faint 

 suggestion of movement. This may answer to the com- 

 mencing seismic movement, but, unlike the other move- 

 inents on the declination trace, it was not noticed until 

 after the Milne record had been examined, and it should 

 be regarded as doubtful. 



The horizontal-force curve showed only one conspicuous 

 burr, the measured time for which was 433. For somi- 



.32 36 40 i,l A.M. 



Kjuat $-t£^Zf, IcfOf 



time prior to 4.23 the edge of the declination curve wis 

 pretty sharp, and apparently no disturbance existed. .\nv 

 magnetic effect originating at the seat of the earthquah'' 

 should have affected the Kew magnets several minutes in 

 advance of the earliest seismic record. Thus in this case, 

 at least, the movements shown on the magnetic curves are 

 of purely mechanical origin — the magnetograph acting as 

 a seismograph — or else they represent some magnetic effect 

 arising in the near neighbourhood on the arrival there 

 of the earthquake waves. 



The periods of the declination and horizontal-force 

 magnets and of the Milni' boom are respectively about 

 't>5, 13J, and 17.1 seconds, and as recorders of earthquakes 



NO. 2045, VOL. 79] 



