Variation in Value of Atmospheric Electrical Potential. 397 
On the following day observations from 5.10 a.m. to 7.30 p.m. gave 
practically the same readings as those recorded in the Table. 
From Belfast to Louren9o Marques observations were taken as 
opportunity offered during the stoppage of the train, but unless a clear 
space some distance to windward from the engine could be found the 
results were valueless. It has been shown in previous papers " how the 
steam from an engine affects the potential gradient. 
The best values are those given in Table II., which were observed 
during long stoppages of the train. 
Table II. 
Belfast to Lourenco Marques. 
Time. 
Altitude. 
Pot. Grad. 
7.30 a.m. 
6,500 
58 
Machadodorp 
8.5 
5,279 
90 
Waterval Boden 
8.30 
4,826 
100 
Waterval Onder 
9.0 
4,144 
110 
9.20 
3,841 
140 
10.29 
2,930 
150 
11.0 
2,410 
180 
11.40 
2,349 
200 
12.20 p.m. 
1,827 
210 
12.50 
1,342 
250 
2.50 
619 
300 
Lourengo Marques 
4.35 
0 
500 
A progressive increase in the potential gradient is seen as the altitude 
diminishes. The numbers given are not those actually observed, but 
numbers calculated as follows : — 
In order to compare the potential gradient at one place with that at 
another, the times of observation should be the same, or rather corre- 
sponding points on the daily curve should be taken. This, of course, 
would have necessitated a stop of one day at each place, and as this was 
impossible the following method was used, which, although obviously 
inaccurate, will not be very far from the truth. The relation between the 
maximum value of the potential gradient and the minimum value, is, 
for Bloemfontein as 4:1, for Pretoria 4 : 1, Belfast 4-5 : 1, Lourengo 
Marques 5 : 1 for the morning observation, and rather less for the after- 
noon. As an approximation the minimum may be taken as occurring at 
* South Africmi Journal of Science, February, 1912. 
28 
