426 



NA TURE 



[October 7, 1919 



hundred thousand amperes — wliich again is rather sur- 

 prisingly moderate, and leaves plenty of margin for under- 

 estimate of disturbance and for local perturbations too 

 great for the instruments. 



I send this little calculation because some doubt has 

 been expressed as to whether the magnetic effect of a solar 

 stream would be adequate to explain observed facts. It 

 appears to me in general outline to be ample, both in 

 amount and kind. Oliver Lodge. 



University of Birmingham, October 4. 



The Magnetic Storm. 



Some details of the magnetic variations during a period 

 including the great magnetic storm of September 25 may 

 be of interest to the readers of Nature. 



From the beginning of the month the magnets were com- 

 paratively quiet, disturbed from a state of calm only by 

 small oscillations, increasing somewhat on September 7, S, 

 and 15 until September 21, when a greater disturbance was 

 recorded between noon and midnight. This was followed 

 by an approximate calm until 8.30 a.m. of September 25, 

 when the coming storm was foreshadowed by a sudden 

 small dip in the curves of the declination and horizontal- 

 force magnets, and a slower fall of vertical-force curve, in- 

 dicating an eastward movement in declination and a 

 diminution of the two components of force. The pre- 



midnight, when the small after-tremors commenced, and 

 continued until about p.m. of September 27. 



The seismologist cannot fail to see in these three phases 

 of oscillation an imitation of the pendular swings produced 

 by a distant earthcjuake, and the preliminary movements 

 aie undoubtedly of the first interest to the student of 

 terrestrial magnetism. The suggestion is very pointed 

 that, whatever be the cause of the magnetic storm, it must 

 be something arriving in our neighbourhood, whether 

 directly from the sun or circulating round it, of which a 

 part travels quicker and has less effect than the sloweK 

 moving particles which produce the great oscillations; but 

 we are in no position to meet the difficulties which beset 

 any definite supposition as to the nature of these particles, 

 and defend it against the apparent contradictions which 

 assail it. \Yalter Sidgreaves. 



September 28. 



THE MEETING OF THE INTERNATIONAL 



GEODETIC ASSOCIATION IN LONDON 



AND CAMBRIDGE. 



"T^HE meeting which began in London on Tuesday, 



-'- .September 21, and ended in Cambridge on 



September 30, has been notable in several respects. 



Great Britain joined the International Geodetic As- 



.-» v'\,,,(VMi 



'i'ti fei 



^iiii 





A 





p. 



Declinalion Magnelogram, Stonyhur^t College Observatory, September 25, 1Q09. 



liminary oscillations thus started remained small, averaging 

 about 10' of arc on both the declination and horizontal- 

 force curves until some time between 11.40 a.m. and noon, 

 whilst the films were being changed. It is, therefore, not 

 clear on which side of the film the spot of light was carried 

 away by the first great deflection ; but in declination it 

 remained off the paper until 11.40 a.m., when it was 

 found returning from the east, while the horizontal force 

 had already recovered, and, together with the vertical 

 fc-rce, was showing a rapidly increasing intensity. 



The general deflection in declination was westward 

 throughout the storm, with many rapid moves off the 

 film, and only two violent throws to the east, one at the 

 commencement and the other at the end of the storm, the 

 latter being a swing across the film from the extreme west 

 edge over the east edge in less than two minutes of time, 

 covering more than 25 degrees. An equally great and 

 rapid movement on the horizontal-force curve took place 

 at the same time near the close of the storm, with 

 increase of the component force, and the vertical-force 

 balance finally heeled over through loss of force. 



The storm seems to have been at its height between 

 3.30 and 5.30 p.m., when the light spots of all three instru- 

 ments were, for the most part, off the papers, and ended 

 at 8.30 p.m. It was followed by smaller rapid oscilla- 

 tions of the declination and horizontal-force needles until 

 KG. 2084, VOL. 81] 



sociation only twelve years ago. The triennial 

 meeting has been held this year for the first time 

 in England, and for the first time Greater Britain 

 has been represented by special delegates from 

 the Governments of India, Australia, and Canada; 

 Chili has becotne a member of the associa- 

 tion and sent a representative ; and the Egyptian 

 Government has been represented by one of the British 

 officers of the Survey Department. The London 

 meeting marks, then, a broadening of the interests of 

 the association on the political and administrative 

 side; on the scientific side it has been remarkable for 

 the extremely interesting reports upon the special 

 problems of the internal constitution of the earth and 

 the lunar earth-tides. 



.About fifty delegates, from twenty nations, were 

 appointed to represent their respective Governments, 

 and came to the contjress, which held its London 

 meetings in the handsimie apartments of the Institu- 

 tion of Civil Engineers, kindly placed at the disposal 

 of the British representative. A large part of the 

 business of the association consists in the reception 

 and adoption of general reports of a highly condensed 

 and technical character, impossible to summarise. 



