JANUAET 13, 1911] 



SCIENCE 



51 



as the result of my analysis of the systems 

 causing the secular variation. 



The principal system, however, involved 

 in the production of the secular variation 

 is still to be revealed, and a promising line 

 of inquiry, at present in progress, is the 

 concomitant study of the laws followed by 

 the secular variation and the lunar diurnal 

 variation of the earth's magnetism; I have 

 found that both follow remarkably similar 

 laws in their distribution over the earth. 

 It might also be mentioned here that owing 

 to the non-commensurability in the periods 

 of the solar-diurnal and the lunar-diurnal 

 variations, there is an outstanding daily 

 residual of the right magnitude for the 

 production of the secular variation. 



Sufficient has been given to show how 

 important and fruitful is the study of the 

 "abnormal" features of the earth's mag- 

 netism. It seems probable that we shall 

 learn more from a close investigation of 

 magnetic disturbances — of the irregular 

 phenomena — than of the normal and regu- 

 lar features. In any event we find that the 

 "abnormal" is such an intimate part of 

 the supposedly "normal" that it seems un- 

 wise really to make a separation. We 

 fully endorse the view of Schuster when he 

 says: 



Outbreaks of magnetic disturbances, affecting 

 sometimes the whole of the earth simultaneously, 

 may be explained by the sudden local changes of 

 conductivity which may extend through restricted 

 or extensive portions of the atmosphere. I have 

 shown in another place that the energy involved in 

 a great magnetic storm is so considerable that we 

 can only think of the earth's rotational energy as 

 the source from which it ultimately is drawn. 



According to the views above set forth, 

 the various manifestations of solar activity, 

 sunspots, protuberances, etc., are not the 

 direct but the indirect cause of the earth's 

 magnetic storms. Their effect appears to 

 be more in the nature of a releasing or 

 "trigger" action, setting in operation elec- 



tric forces already in existence in the upper 

 regions of the atmosphere; terrestrial 

 sources, in reality, however, supply the 

 energy required for the magnetic storm. 



THE EABTH'S permanent MAGNETIC FIELD 



Our studies began with magnetic dis- 

 turbances, and we soon found that we 

 were dealing with systems of forces re- 

 markably similar to those composing the 

 earth's permanent magnetic field. Given 

 an existing electrical field in the upper re- 

 gions, it follows at once, from our knowl- 

 edge of the necessarily varying conductiv- 

 ity of the atmosphere resulting from solar 

 radiations of various kinds, that this field 

 must be an exceedingly variable one. 

 First, it must be subject to a daily varia- 

 tion of an average normal kind correspond- 

 ing to the average normal solar radiation, 

 and superposed on this more or less spas- 

 modic fluctuations, which represent the 

 variability in the supply of the essentials 

 in the solar radiations for producing the 

 observed magnetic effects. 



In this connection let me point out an 

 interesting bit of evidence furnished dur- 

 ing the time of the total solar eclipse which 

 occurred in the United States in May, 

 1900. As the result of the special mag- 

 netic observations, made chiefly by the ob- 

 servers of the United States Coast and 

 Geodetic Survey, a small magnetic pertur- 

 bation revealed itself at each station along 

 the belt from Georgia to Maryland. This 

 perturbation did not begin according to 

 absolute time nor according to local mean 

 time, but bore a distinct relation to the 

 time of passage of the shadow cone. It 

 was thus shown that by the interposition 

 of the moon between the sun and the earth, 

 certain radiations were cut off as the re- 

 sult of which a magnetic fluctuation was 

 produced. I recall that the late Professor 

 Newcomb appeared rather skeptical as to 



