188 



ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



of action at times stationary but generally moving from place to 

 place. Storms of the first type may follow comparatively quiet or 

 normal conditions and are often initiated by a sudden sharp shift — a 

 "sudden commencement" — simultaneous, within a minute of time, 

 at all observatories. A tendency to oscillation and continued unrest 

 during intervals varying from 10 hours to several days is one of the 

 outstanding features of such storms. 



The second, more local, type of transient magnetic disturbances 

 is usually represented by a rather slowly developing "bay" on the 

 record extending over half an hour or somewhat more. 



ZI-KA-WEI 



AGINCOURT 



13 14 15 I 



16 'H5 14 15 



or loor 20oy 



SCALE '' ■ ' ' '' ''' ! '''' ' '' ■•' 



FiQUBE 6. — World-wide sudden commencement of geomagnetic storm at 13''09™, 

 Greenwich mean time, May 13, 1921, showing changes in horizontal-intensity 

 component. 



Thus disturbed or quiet magnetic conditions nearly always affect 

 the whole earth simultaneously. The possibility of any connection 

 between the disturbances of the earth's magnetic field and the weather 

 is excluded since weather is so distinctly local and so different all 

 over the world. But polar lights — the auroras — are always seen in 

 polar regions when magnetic storms occur. In 1741 Celsius and 



