782 Mr. Charles Chree [March 4, 



pioneer of magnetic work in Scotland and India, believed his observa- 

 tions to indicate a period of abont 20 days. From an elaborate study 

 of many years' storms at Greenwich, Mr. E. W. Maunder deduced a 

 period of 27 "275 days, and Mr. Arthur Harvey independently 

 from a study of storms at Toronto deduced the remarkably similar 

 period of 27 * 246 days. The latest result of this kind is due to the 

 eminent German magnetician already mentioned, Dr. Schmidt, Avho 

 believes in a period of 20-97 days. Schmidt found evidence of 

 this period in a number of recent storms, and he declares that it 

 exists in the case of very large storms even when separated by many 

 years. He found that the dates of occurrence of 5 out of the 7 

 largest storms recorded at Potsdam (see Table IX.) could be deduced 

 to a high degree of accuracy from the expression 



2410000 + ;^031-o + n x 29*97, 



wliich counts time in days from the commencement of the Julian era. 

 The degree of accordance of the calculated and actual dates is sliown 

 in Table X. 



Table X.-— Schmidt's Storm Period Formula 2410000 + 3031 -Q+nxSQ- 97. 



Patting n 



Calculated date of storm, 3031 • 3061 • G417 • G 

 Actual ,, „ 3030 3061 6419 



185 



7614-4 8575-4 

 7616 8575 



The eminence of Dr. Schmidt as a magnetician, and the wide ex- 

 perience of Mr. Maunder in sun-spot work, alike demand our respect. 

 But a consideration that must influence one's judgment is that if one 

 of these two gentlemen is correct, the other must apparently be some- 

 wliat badly wrong, unless we admit tivo distinct periods. 



§ 16. A slide which serves as a chronicle of magnetic history at 

 Kew from August 20 to November 16, 1909, Avill illustrate some of 

 the difficulties in the way when one attempts either to prove or dis- 

 prove the existence of a period in magnetic storms. 



The upper curve shows the value each day of the absolute decli- 

 nation range at Kew, the lower the value at each midnight of the 

 horizontal force. In the absence of magnetic disturbances, the decli- 

 nation range would presumably vary only with the season, the change 

 from day to day being insignificant, and similarly we should expect 

 the horizontal force to have a nearly constant value at each midnight 

 of a month. Instead of this we see incessant variations from day to 

 day, and the numl^er of days in which the declination range conspicu- 

 ously overtops the average is considerable. During these days there 

 is usually a distinct fall in the horizontal force, a circumstance also 



