Failure of the Yan Yean Reservoir. 



143 



velocity, as, according to the usual fbrmulte it would be very 

 small or difficult to compute. 



It is thus very important to notice that they assume the six 

 winter months discharge at eight times the amount of the six 

 summer months, while the river, when level with its banks, 

 can only contain three times the amount. 



They also assume that the river is really level with its 

 banks, although they have never seen it in the winter months; 

 but they are quite satisfied on this point from the evidence 

 of a resident farmer. 



This, it will be observed, is not very scientific or reliable 

 evidence to check their calculations, or to justify the expendi- 

 ture of £650,000 of public money, and accordingly the 

 farmer's son, an intelligent lad of nineteen, in his father's 

 absence, told Mr. Wekey and Mr. George Wilkie, that the 

 river was only half full during the winter months, except in 

 floods, which, with the same velocity, exactly accords with 

 my own estimate of the winter discharge. 



The principle upon which Mr. Dempsey's tables are based 

 is precisely the same as that adopted by Dr. Dalton and Dr. 

 Thomson, for ascertaining the watershed. He calculated 

 that 57 '6 per cent, of the rain is evaporated, and 42*4 remains 

 to supply springs and rivers. 



Whether Mr. Dempsey's tables are the result of his own 

 experiments, or those of others, he does not say, but he 

 clearly states that the evaporation mainly depends upon the 

 temperature, heat promoting it, cold retarding it, and 

 therefore I cannot conceive that he would_ commit such 

 an egregious blunder as to apply his tables, without correc- 

 tion for the great difference of temperature, to determine 

 the proportion of the rain evaporated in this colony. 



In calculating the evaporation from the surface of water 

 here from English tables, I assumed for each month the 

 evaporation of a corresponding month in England,^ with the 

 same or a less mean temperature, and I thus obtained eight 

 feet two inches, which is sufficiently near to Dr. Davy's es- 

 timate of nine feet to show the correctness of the method. 



I therefore hold it to be scientifically correct to adopt the 

 same method with Mr. Dempsey's tables in order to deter- 

 mine the proportion of rain that is evaporated from the 

 ground. 



I have drawn up three tables for the purpose of illustrat- 

 ing the contrast between the proportion of rain evaporated 

 here and in England, allowing the same proportion to the 



