STUDIES IX ATMOSPHERIC ELECTRICITY. 3 



occurrence of the extreme \^nlues in each monthly mean, we have the 

 accompanying- table. 



Tluis we see that the morning maximum occurs a little more 

 than one hour after sunrise, throughout all the seasons. The fact 

 that this maximum occurs much more regularly than the other 

 extreme v;ihies, is not confined to the monthly means, but also 

 exhibits itself in daily values. For, it generally deviates only an 

 hour Ol- tw(3 from one day to another, while the evening maximum, 

 for instance, ma}^ occur at 7 p.m., or not be reached even at midnight. 

 This fact is at least one of the causes which make the mornin»- 



o 



maximum so conspicuous in the diurnal variation curve in each 

 month. 



THEORIES. 



]>efore proceeding further to examine the relations of the 

 potential to other meteorological phenomena, it will be well here to 

 recapitulate briefl}^ some of the theories of atmospheric electricity, and 

 to state the author's view in accordance with which be wishes to 

 discuss those relations. 



Perhaps the oldest of the theories still held is that of Erman 

 (1803) and Peltier (1836), which supposes the earth to have an 

 inherent negative charge. Among the modern supporters of this 

 theory we find Exner, who, while ascribing the primary cause of 

 atmospheric electricity to the earth's inherent charge, strives to 

 explain its variations as being due to the action of the aqueous 

 vapour, which in evaporating from the earth's surface, carries more or 

 less negative charge with it," the latter supposition being founded on 



1) Exner, Sitzungsberichte d. Wiener Akad. d. W. 93. p. 222. 188'3 ; 93. p. 419. 1837; 97, 

 p. 277. 188S. 



