PRESIDENTIAL ADDRESS SECTION A. 



The experiment was commenced on the ist January, 1919, 

 and the results are shown graphically on Plate 3 for a i^eriod of 

 18 days. The daily evaporation, at the Union Observatory, for 

 Ihe same period, is also shown on the chart. Readings were taken 

 every half-hour during the 24 hours of each day, and the 

 necessary corrections were made for rainfall- The experiment 

 definitely shows that there is a considerable increase in evapora- 

 tion as the temperature of the w^ter increases, and the results 

 are shown below : — 



Daily Evaporation for each 

 Temperature from 1'^ F. rise in 



Temperature. 



67.31° F. to 73.36° F. 

 73.36° F. to 82.71° F. 

 82.71° F. to 90.44° F. 



0.013 inches. 

 0.018 inches. 

 0.023 inches. 



The actual evaporation in inches during the 18 days is f^xven 

 below : — 



Average Evaporation 



Temperature in 



Fahrenheit Inches. 



Remarks. 



Tank at Union Observatory 



(Johannesburg). 

 Village Pumping Station Tank. 



Do. Do. 



Do. Do. 



It will be observed from the above figures that for a rise in 

 temperature of about 23 °F. the evaporation increased from 3.28 

 lo 10.92 inches in 18 days. Evaporation is also affected by many 

 other factors, such as wind, humidity, etc., but it is unnecessary 

 to discuss these factors in this paper. 



If, therefore, the natural temperature of a river is raised 

 owing to water being used by Power Stations for condensing 

 purposes, it follows that the loss due to evaporation in the river 

 is considerably increased. Based upon an increase in tempera- 

 ture oif i8°F. (72.5 to 90.5), and taking an area of one square 

 mile of river, which represents a stretch 11.7 miles long by 450 

 teet wide, the additional loss above that due to natural evapora- 

 tion is 5,338,000 gajlons per day- 



A case in point has been recently considered in connection 

 with the Rand Mines Power Company's station at Vereeniging, 

 when it was clearly shown before the Parliamentary Select Com- 

 mittee that the extra loss from the Rand Water Board reservoir, 

 due to condensing water being returned to the river, would be 

 about 300 million gallons per annum if the circulating water for 

 condensing was i8°F. higher than the natural temperature of 

 the river water. 



It was also found during the investigations that the natural 

 temperature of the river water at 9 o'clock in the morning is. 

 approximately equal to the mean temperature of the air during 

 the day. 



