1910] 



on Magnetic Storms. 



•81 



sudden commencement is here represented by a very large oscillatory 

 movement. This oscillatory character was prominent in all the 

 sudden commencements shown in the Antarctic curves. It represents 

 a phase of which faint traces are sometimes seen in temperate 

 latitudes. 



§ 1?>. The next slide show^s the Kew horizontal force record of a 

 disturbance which began suddenly near 2 p.m. on November 18, 1894, 

 It illustrates a second curious, but very common, phenomenon, viz. a 

 fall in the value of the horizontal force which survives the storm. 

 This is a good example of an ordinary disturbance in which the 

 magnetic changes are of consideral)le size, but show fcAv large oscilla- 

 tions. 



§ 14. The next three slides refer to a disturbance of a very different 

 type, viz., the great storm of September 25, 1909. The first shows 

 the record obtained at the 01)ser\atory of the Jesuit Fathers at Ja\- 

 kia-pang in China. The disturbauce is large, but the energy apparent 

 is small compared Avith tliat indicated l)y the next two slides, which show 

 the declination and horizontal force traces at Kew. The storm there 

 and at other European stations was remarkable for its large and rapid 

 oscillations. The movements were so rapid that the photographic 

 paper was not sensitive enough to show some of them clearly. Some 

 of the record was also lost through the trace going off the sheet on 

 both sides. The storm was of comparatively short duration, but 

 while it lasted, it displayed an energy unrivalled at Kew during the 

 last 20 years. The only previous Kew records comparable with it 

 which I have seen date from 1859. 



Dr. Schmidt, the leading Gei-man autliority on our sul)ject, 

 assigns to this storm the first place of all I'ecorded since the Potsdam 

 Observatory came into existence some 20 years ago. Table IX. gives 

 his estimate, on an arbitrary scale, of the intensity of the seven 

 laro^est storms that Iiua'c been recorded at Potsdam. 



Table IX. — Dr. Ad. Schmidt's Estimate of Intensity of 

 jNIagnetic Stor]\[s. 



Date of storm. 



September 25, 1909 

 October 31, 1903 

 February 14, 1892 

 July 20, 1894 



Disturbance 

 Potsdam. 



3800 



2860 



over 1800 



1580 



Date of Storm. 



Disturbauce at 

 Potsdam. 



September 11, 1908 1520 

 August 20, 1894 . 1410 

 February 9, 1907 . 1340 



§ 15. An old question which has received a good deal of recent 

 attention is wdiether there is or is not a cyclic period approaching a 

 month in the occurrence of magnetic storms. J. A. Broun, an early 



