238 



The Data on which we have to depend 



longer availablej although it forms a portion of the rainwater 

 that is shed from the drainage area. Therefore the expres- 

 sion available rainfall very incorrectly conveys the meaning 

 that is intended^ and is of little use in any scientific inquiry. 



To obviate this difficulty I have made use of the term 

 *^ watershed " to signify the proportion of the rain that is 

 shed from any given area or tract of country. This term has 

 been ordinarily applied to the area or tract of country that 

 sheds water, but this is so gross a corruption of the analogies 

 of the English language, that I have avoided using it in this 

 sense. 



At the time that my paper on the Tallure of the Yan 

 Yean Kesei-voir Avas published, I had only heard Mr. Hodg- 

 kinson's paper read, and, on its subsequent publication, I was 

 much surprised to find that I had misunderstood him in 

 several important points. 



While, therefore, I still regard his paper as a valuable 

 contribution to practical science, I regret extremely to add, 

 that I am compelled to differ from him very materially in 

 some of the scientific data upon which he bases his con- 

 clusions. 



Mr. Hodgklnson himself admits the necessity of throwing 

 more light on the question of evaporation, "in order that 

 definite conclusions relative to the probable supply of water 

 derivable from Yan Yean might be arrived at;" and the 

 extraordinary discrepancy of opinion that prevails on the 

 subject clearly shows that, until this question is determined 

 by scientific investigation, no definite conclusions can be 

 arrived at. 



I proceed now to consider what are the grounds upon 

 which Mr. Hodgkinson bases his confident opinion, that there 

 will be a very abundant supply for a population nearly three 

 times greater than the present population of Melbourne, 

 while I have myself found, by a strict method of investiga- 

 tion, based on actual measurements of the river, that, after 

 deducting the loss that is at present sustained from evapora- 

 tion in the marshes, and the probable loss from evaporation 

 in the reservoir, according to Dr. Davey's estimate, there will 

 be no water for any population. 



Mr. Hodgkinson's theoretical estimate of the watershed of 

 the Plenty basin differs very little from my own. In dedu- 

 cing the watershed from English data, I assumed four-and-a- 

 half inches as the nearest approximation, while he assumes 

 five inches. The difference therefore amounts to half an inch 

 or one-tenth, which is equal to one foot in the reservoir. 



