724 Earth- Air Electric Currents. 



In fioc. 2 the mean values for the ten days are plotted. In 

 the figure the mean value during the hour is referred to the 

 middle instant of the hour expressed in local time. Thus 

 the value for the hour 10 to 11 I.S.T. is plotted against 

 10 hrs. 9 min. local time. 



-big'. 2. 

 I Z 3 4 5 6 7 8 9 10 II noon 15 14 15 16 17 18 19 20 21 22 23 mot 



It has already been pointed out that the exposure of the 

 plate was not satisfactory, so that the absolute values are 

 uncertain; but it is interesting to note that the mean value 

 found for the conductivity of the air, viz. 5*8 x 10~ 4 els. 

 unit, agrees very well with that found by other workers, all 

 of whom have found values of the order of magnitude of 

 10~ 4 els. unit : see the list given on page 83 of H. Mache 

 and von Schweidler's book Atmosplierische Eleldrizitat. 



Turning now to the daily variations which are much less 

 affected by the unsatisfactory exposure, the curves show that 

 the potential gradient had its chief minimum and maximum 

 in the early hours of the morning and evening respectively. 

 The conductivity was above the average during the night 

 and early morning, and below during the day and evening. 

 During the period over which the observations extended the 

 sun rose above the surrounding hills soon after 7 hrs. local 

 time and set at about 17 hrs. Thus it would appear as if 



