130 



Failure of the Yan Yean Reservoir. 



But how could sucli a flood be secured for the reservoir? 

 The same amount of water would take twelve days to pass 

 through the aqueduct at one mile an hour, which is the high- 

 est velocity that it would be either safe or prudent to allow. 

 A higher than this, in such a winding canal cut out of the 

 clay-slate, would convert the water into mud, and break up 

 the sides, and wash away the artificial banks ; and for the sake 

 of such a flood, which may possibly occur once in ten years, 

 would it be reasonable to spend £30,000 in enlarging the aque- 

 duct to four times its present size? and without suitable 

 embankments raised at an enormous cost, to dam up such a 

 flood, one half would not enter the aqueduct at all. 



As twelve inches of rain fell during the month, instead 

 of four, we may safely conclude that this flood resulted from 

 a rainfall of at least six inches, which would give thirteen 

 feet in the reservoir, so that out of thirteen feet of rainfall 

 in one of the heaviest floods on record, only three feet reach 

 the rivers, or less than one-fourth. 



The only other source of supply to be noticed is dew. In 

 England from four to five inches have been computed as the 

 amount of dcAV deposited on the ground, but I am not aware 

 of any experiments to show the amount deposited on water. 

 I feel persuaded that the atmosphere is generally so dry here, 

 that the amount of dew must be very small ; and unless in 

 the case of very shallow pools and lakes, there can be very 

 little deposited on water. A depth of two or three feet will, 

 in a great measure, prevent the formation of dew, because as 

 the upper particles become cooled they at the same time 

 descend, from their increased density, to make way for the 

 warmer and hghter particles underneath, and until the whole 

 depth of water has attained a considerably lower temperature 

 than the atmosphere with which it is in contact, no deposition 

 of dew can take place. 



As it is possible that there may be some dew, when there 

 is very Httle water in the reservoir, I shall on this account 

 allow an increase of two inches for the whole surface, which 

 is equal to sixteen and one-half days supply for the city. 



The whole amount will stand thus: — 



From the River Plenty - - - ^ ft. 6^in. 



Floods in ditto - - " ■ " i l 

 Increase from winter rain " ' " o a 



Eainfall in reservoir - - - "12 

 Drainage area of ditto - - - j; „ 



Dew ^ ^ 



Total amount H 



