Discussion of Atmospheric-Electric Observations 



419 



indicated by the thermograph records, shows a maximum about the same time as the 

 maximum in the curve for n+, which fact is in harmony with the above view. The average 

 daily range in temperature is about 5°C.,and this corresponds to a range of about 5 per cent 

 in a and consequently to about 2.5 per cent in n+. Apart, however, from these considera- 

 tions with regard to the variation of a, Simpson, in his experiments at Karasjok,* found, as 

 an experimental fact, that the ionic content increased by about 35 per cent for 20 degrees 

 increase in temperature. Such a change, which is considerably larger than that calculated 

 from the change in a to be anticipated from laboratory experiments, is in itself almost 

 sufl&cient to account for the present diurnal variation in n+ as a pure result of the diurnal 

 variation in temperature. 



Fio. 27.— Comparison of the Potential-Gradient Diurnal- Variation Curve for the Ocean with Those for Various Land Stations. 



The diurnal variation of the ionic content on land is influenced much by the local 

 conditions. Gockel,^ from measurements made at Freiburg, finds a maximum for n+ at 

 2 p. m. in winter and at 4 p. m. in the summer. These results are in general agreement 

 with those of the present cruise, except that they give the maximum somewhat later, a 



^PUl Trans. R. Soc. A, vol. 205, p. 79, 1905. 



^Met. Zeit., vol. 23, p. 53, 1906. 



