402 
Transactions of tlw Boyal Society of South Africa. 
Observations at Johannesburg. 
In July, 1911, some casual observations were taken of the potential 
gradients at Johannesburg and the value found to be much higher than 
that at Pretoria and Vereeniging, places about 45 miles from Johannes- 
burg, and at a lower altitude, and also some considerable variations were 
observed in the values at different points outside the town. Very large 
quantities of steam and smoke are emitted from the engines at the mines, 
and these must have some influence upon the potential of the air in the 
neighbourhood. Another factor which has to be taken into account is 
the cloud of fine dust which rises from the mine heap during the process 
of "tipping" and also caught up by the wind. Steam gives to air a 
positive charge, whilst dust gives a negative. 
Observations were begun at the Observatory on September 25th at 
5 a.m. and continued until a steady value appeared to be reached, and 
the results are given in Table VII. 
Table VII. 
Johannesburg, September 26th. 
Time. 
Pot. Grad. 
Time. 
Pot. Grad. 
Time. 
Pot. Grad. 
5.0 a.m. 
60 
6*25 a.m. 
100 
7.55 a.m. 
160 
5.10 
66 
6.30 
100 
8.0 
180 
5.12 
70 
6.40 
105 
8.20 
150 
5.30 
75 
6.50 
115 
8.30 
100 
5.45 
80 
7.0 
125 
8.40 
155 
6.0 
80 
7.10 
150 
9.0 
155 
6.10 
98 
7.15 
160 
9.20 
160 
6.17 
95 
7.20 
168 
9.30 
160 
6.20 
95 
7.30 
168 
9.45 
180 
10.0 
168 
10.20 
168 
1 
11.40 
160 
It may be seen that the variation is not so great as that observed in 
the open country or in such places as Bloemfontein or Lourengo Marques. 
At 10.20 a journey was made to Norwood, about 3 miles to the north 
of the Observatory, and the gradient taken there. Its value was only 52, 
and on returning to the Observatory the value there was 160, and kept at 
