138 



Failure of the Van Yean Reservoir. 



per head, the difference between these estimates of eva- 

 poration would supply 123,000; deduct this from 190,000, 

 and we have 67,000 as Mr. Ilodgklnson's estimate, accord- 

 ing to this higher rate of evaporation. 



My own estimate, deducting the same evaporation, would 

 supply 43,000. 



Kegarding these estimates as mere approximations, it will 

 thus be seen that the difference between Mr. Hodgkinson's 

 and my own is very small, being equal to only ten inches 

 in the reservoir. 



Now, while the greatest importance is to be attached to 

 Mr. Hodgkinson's estimate of the watershed of the Plenty, 

 as it is founded on a thorough knowledge of the river, and 

 much practical experience both here and in England, I feel 

 inclined to prefer Dr. Davy's estimate of the evaporation, 

 and from measurements which I recently made on a larger 

 sheet of water than Mr. Hodgkinson's pond, I found that in 

 twenty-eight days the evaporation exactly equalled eleven 

 inches, or 0'39 inches per day, which would give very nearly 

 nine feet in twelve months. 



But, after all, no scientific result can be obtained from 

 ponds and Avaterholes, unless they are water-tight, which it 

 is impossible to ascertain. 



Thus, with Dr. Davy's lowest estimate of evaporation, Mr. 

 Hodgkinson's estimate of the watershed would supply 

 94,000, so long as there are no droughts; but Dr. Davy 

 gives us very little hope of escaping from such calamitous 

 visitations. 



Mr. Hodgkinson likewise expresses a very unfavourable 

 opinion with respect to the quality of the water, when stored 

 in the reservoir^, and thinks that the Plenty is more likely to 

 suffer deterioration fi om a resident population than the Yarra. 



It is interesting and highly important in this inquiry to 

 know that Mr. Hodgkinson's estimate of the watershed and 

 my own are still further corroborated by the investigations 

 of Dr. Davy. 



He is not sufficiently acquainted with the geological 

 character of the Plenty basin to give any exact estimate of 

 the watershed, but, from observations and calculations which 

 he has made respecting the force of evaporation from the 

 surface of the ground, in this colony, he regards one-eighth 

 of the rain, over any large area of surface, as the most that 

 can reach the rivers, under any conditions of slope or geo- 

 logical formation, and he thinks that in many districts the 



