80 
Transactions of the Boyal Society of South Africa, 
6 p.m. This gives the usual range of time for adust-storm in this country 
but during the past year they frequently went on during the night. (10) 
is for December 22nd. (11) for December 5th records rain, but no 
dust. (12) for December 15th records the passage of two short storms. 
That at 4 p.m. was of short duration, but the positive potential was 
reduced some time before the dust reached the observatory, and the 
negative value remained high for some time after the storm had passed 
and the air had become clear. It may be noted also that there was a 
very rapid rise in the positive potential before the coming of the storm. 
The time of passage of the dust over the observatory was not more than 
three minutes, but the negative charges persisted afterwards for 40 
minutes. This persistence has often been observed, but a negative charge 
has never been registered unless the dust has been near enough to 
be visible. 
SUMMAKY. 
(1) Observation taken on the daily range of the atmospheric potential 
gradient on the high veldt," show that considerable variations may occur 
as a consequence of the presence of dust in the atmosphere. 
(2) The effect of the siliceous dust of South Africa is to lower the 
normal positive gradient, and if present in sufficient amount to reverse it. 
(3) Wind unaccompanied by dust has very little influence on the 
normal charge in the atmosphere. 
(4) The rain which fell during the period under observation was 
invariably charged negatively. 
My grateful thanks are due to The Eoyal Society of South Africa for 
providing the funds by means of which the apparatus for taking these 
observations was purchased. 
* This is true also for other places in South Africa where the matter was tested. 
