56 Transactions of the South African Philosophical Society, 



not possible to vary either of the conditions without introducing 

 variations into the others. As Tyndall truly remarked, "■ the essence 

 of good experimenting consists in the exclusion of circumstances 

 which would render the pure and simple questions which we intend 

 to put to Nature, impure and composite ones ; " * and for an 

 illustration we have the case cited above of the rate of evaporation 

 varying in its annual march with the velocity of the wind — the latter, 

 as it will be remembered, also varying with the annual variation 

 in the dryness of the air. No doubt the complexity of the 

 meteorological law governing the rate of evaporation from a water 

 surface must be accounted responsible for the illusory shape it has 

 assumed in the hands of some theorists. 



Occasional experimentalists have resorted to laboratory practice in 

 order to surmount some of the inherent difficulties of the native 

 problem. A typical instance may be quoted: ** In my first experi- 

 ments out of doors," says Mr. J. E. Mann, " I found that the 

 amount of error in measuring the small variations of short periods 

 prevented me obtaining any accurate idea of the fluctuations, in the 

 amount of evaporation, that followed the constant and incessant 

 alteration in the state of the atmosphere and the temperature of the 

 water. I therefore looked to artificial means for obtaining a greater 

 amount of evaporation under more fixed conditions ; and having 

 obtained some certain basis, I endeavoured therefrom to devise a 

 rule by which the amount of evaporation going on under natural 

 conditions might be arrived at. After many failures, I adopted an 

 arrangement which I think gave satisfactory results. An ordinary 

 evaporating vessel, about 8 inches in diameter and IJ- inches deep, 

 containing a measured quantity of water, was floated in a larger 

 vessel of water ; the large vessel was placed upon an ordinary gas 

 stove, which gave the means of regulating the heat in the water 

 contained in the evaporating vessel to a considerable nicety, and of 

 retaining it for a length of time at one degree of temperature."! 

 It is at the least to the credit of the gas stove that with a w^ater 

 temperature of 192°, an air temperature of 65°, and a humidity ratio 



* Ilent a Mode of Motion. Lecture xiii, 



f Fi'oceedingH of the Meteorologiad Socictij, vol. v., 1871. Mann gives the 

 following formula : — 



(4(JG6E)- = G-T j 1— {^\ "'' I 



In which E is the depth evaporated in inches per hour, G the weight in grains of a 

 cubic foot of vapour due to the temperature of the water, T the absolute temperature 

 of the evaporating surface, e the elastic force of the vapour in the air in inches, and 

 c, the elastic force of vapour due to the water temperature. 



