ATMOSPHERIC ELKCTKICITV. 



235 



The curve shows how the potential gradient varied as one 

 receded up the river from the fall. It cannot very weU be drawn 

 to scale, as the highest point of tlie curve above the zero hne 

 would have to be 250 times the height of the lowest point below 

 the line. The Zambesi river labove the Falls is about 3,000 feet 

 above sea-level. 



Between Bulawayo and Potcliefstroom, the general level of 

 the country does not vary much, and the potential gradient was 

 also uniform and of the order of 150 to 160 volts per metre. At 

 Rhodes's grave and at Johannesburg the values were about 40. 

 In January of this year observations were taken during a journey 

 to Cape Town, with the general result that a steady increase of 

 the potential gradient was observed as the elevation diminished. 



Fig. 2. 



Over the veld and Karoo, the value was about 150; at \\'orcester 

 it rose to 170. and this at a later hour, when the potential is 

 usually lower. 



Cape Tozcii Obsc):'atioiis. — Observations were taken at 

 various points on the coast between the Docks and Camps Bay. 

 The general value of the potential gradient was high, going up 

 to over 500 volts per metre in some instances, but very great 

 fluctuations occurred, owing to a combination of the effects of 

 sun, cloud and wind. 



The following are some typical cases : — 



January 8th : Bright sun, clear air, no cloud ; at 20 feet above 

 sea-level the potential gradient was 550. 



Janmr^^ 9th : Misty, clouds beloiw top of Lion Rock, but 

 Rock visible above the clouds ; potential gradient was 225 volts 

 per metre. x'\s the cloud rose, the top of the Rock became 

 covered, and the potential fell to about 100. On the cloud rising 

 further or. rather, moving away, the potential fell still further. 



