Evaporation Results in New South Wales and South Africa. 9 



and numerical context in the body of the Eeport, the local record of 

 what is a most important branch of knowledge in any country, 

 whether applied to the determination of the strength of levees 

 or dykes, or in deciding as to the volume of water available for 

 storage. 



In addition to the voluminous data respecting rainfall, &c, from 

 which the maps have been constructed, the Eeport gives a few 

 tabular statements of " Evaporation Eesults," as they are called, at 

 different stations, and I purpose making comparisons between some 

 of these results and some that I obtained a few years since at Van 

 Wyks Vley. The observations from which these " results " have 

 been deduced were with one exception taken in evaporation tanks, 

 4 feet in diameter, 3 feet in depth, and sunk into the ground to 

 within an inch of their upper rims. Measurements were made to 

 the top of a rod of brass (rising from a glass float), by the aid of a 

 " contact-gauge " reading to the thousandth part of an inch. 



Mr. Eussell points out that the evaporation from the tank at Lake 

 George, which is at the margin of a large sheet of water, might be 

 expected to be less than that at any of the other stations, and 

 justifies the " expectation" in a short comparative table. There is 

 here a slight apparent anomaly, for the gauge at Sydney " with the 

 waters of the harbour on three sides of it " shows a loss of 34 inches 

 against 29 inches from the Lake George tank, but this is partly, 

 perhaps sufficiently, .explained by the differences in mean shade 

 temperatures, that at Sydney being 62-9°, against that at Lake 

 George only 57*4°. Some part of the difference is, however, 

 possibly due to the difference of elevation above the sea level. 



Lake George, at an elevation above sea level of 2,267 feet, is the 

 one exceptional station at which measurements of loss have been 

 taken on a large water surface, and from these measurements it is 

 found that the lake loses from 50 to 60 per cent, more than the tank 

 does. The experiments have been conducted over a period of eight 

 years, and Mr. Eussell arrives at the important conclusion that — " We 

 must then, I think, assume that the evaporation at Lake George in 

 a dry year, when the lake receives no drainage, is the best guide to 

 the amount lost from evaporation, plies spray carried away by the 

 wind." 



Percolation from the bottom of the lake is not taken into account 

 " for two reasons," says the author ; " first, there is as yet no data, 

 and, secondly, the lake has been full for so many centuries that 

 percolation seems improbable." 



I am, with all respect to Mr. Eussell, inclined to challenge the 

 value of this last assumption, and, on the other hand, I should like 



