ATMOSPHERIC ELECTRICITY AT P.LOEMFONTEIN. 263 



constant until sunrise, as shown by the curve given in the paper 

 referred to. 



The chief factors which modify the electrical charge are 

 mist and dust, from natural causes, and steam and smoke from 

 what may be termed causes due to civilisation. Attention has 

 been paid to these factors in the course of the work, and their 

 general effects tare stated bdlow. The nonnal electrification of 

 the air is positive during fine weather. 



Dishirhing influence. Effect on -\- charge. 



Mist. Increases its value. 



Dust. Diminishes ,, „ 



Steam. Increases „ ,, 



Smoke. Increases „ ,, 



Rain. Decreases ,. „ 



It miust be pointed out that rain is sometimes positively 

 charged, but in this comitry it is generally negative. 



The effects of steam and smoke are noticeaible only in towns 

 or near railways, but the other three factors make their influence 

 fdt anywhere. The effect of dust is more marked than that 

 of mist. 



A day's observation will indicate how the value of the 

 electrification is changed by the various distui^bing factors : — 



Potential Nature of 

 gradient. Charge. 



Just 'before sunrise, slight mist. ... 50 -{- 



Just after sunrise, slight mist .... 58 



7.30 Thick mist 400 + 



8.0 Thick mist 200 -j- 



8.30 Mist clearing 68 -|- 



9.0 Sky clear 85 -|- 



9.30 60 -j- 



lo.o 40 -j- 



10.^0 



ii.b 38 + 



4-0 .._ , 38 + 



5.0 Wind from N.W., accompanied 



with dust 50 — 



5 to 6 Intermittent clouds of dust . . over 400 — 



6.30 Wind dropped . . . : 50 — 



6.40 Wind dropped 



6.50 Rain falling over 400 — 



7.0 Rain falling over 400 — 



7.10 Rain falling over 400 — 



7.30 Rain ceased 60 + 



8.45 40 + ■ 



10.15 4.S + 



IT.O 58 + 



11-30 45 + 



f2.o 45 + 



From It a.m. to 4 p.m. the value was practically constant. 



