DEPARTMENT OF TERRESTRIAL MAGNETISM. 



355 



curves for the three oceans on this basis. The results are shown in the 

 accompanying figure. 



MEAN DIURNAL VARIATION OP POTENTIAL GRADIENT 

 PROM OBSERVATIONS ABOARD THE "CARNEGIE" 

 (Accordtng to Greenwich Civil Mean Time) 



Fig. 1. 



From a comparison of these curves it appears that the daily maximum (like- 

 wise the minimum) of the potential gradient, over the oceans, occurs approxi- 

 mately simultaneously in all localities. The curve in the lower part of the 

 figure represents the mean diurnal variation of the potential gradient as de- 

 rived from the entire 45 series. The times of maximum and minimum as 

 given by this curve (7 p. m. and 4 a. m., G. M. T.) probably represent only an 

 approximate yearly average. 



However, it is significant to note that (a) in western Europe, where local 

 time does not differ greatly from G. M. T., the occurrence of the chief daily 

 minimum at approximately 4 a. m. is common to many stations; (6) the diur- 

 nal variation of the potential gradient as observed at practically all high-lati- 

 tude stations and, in winter, at various stations in temperate latitudes, is 

 very similar to what has here been found for the ocean, when differences in 

 local time are taken into account. It is also significant that Mache and 

 V. Schweidler^ long ago called attention to the fact that only the phase angle 

 of the 12-hour Fourier wave has roughly the same value for most stations, 

 whereas the phase angle corresponding to the 24-hour wave varies greatly 

 from station to station. 



A fact of considerable interest is that the diurnal-variation curves for the 

 potential gradient derived from the Carnegie observations are very similar to 

 curves which represent the diurnal frequencies of the aurora borealis, as 

 observed at several European stations, and also to curves representing the 

 diurnal distribution of certain classes of magnetic disturbances, when all are 

 referred to the same time-basis. It may also be pointed out that owing to the 

 non-coincidence of the Earth's magnetic axis with its axis of rotation, the time 

 of daily potential gradient maximum, as indicated by the ocean curves, 

 corresponds approximately to the time when the Earth's north magnetic pole, 

 for example, is nearest to the Sun, while the daily minimum occurs, in a 



*H. Mache und E. v. Schweidler, Die Atmospharische Elektrizitat, p. 27, Braunschweig, 1909. 



