Failure of the Yan Yean Reservoir, 



149 



wonld be very little increased. Thus the section could not 

 carry three times its present volume without flooding its 

 right bank. 



Such a state of things could scarcely escape the notice of 

 an intelligent resident gentleman^ if it were to go on for 

 eight months in every year^ and yet the Committee of the 

 Society tell us that there is not only three times but ten times 

 the above measurement during eight months in the year. 

 Nor is there the slightest water-worn appearance on the banks 

 to indicate that they form the ordinary channel of the river 

 for any portion of the year. 



I cannot at present furnish^ on reliable authority, the ave- 

 rage increase of rivers in floods; besides, this increase must 

 bear a constant relation to the rainfall, and therefore to the 

 latitude. 



The Committee entertain the opinion that, during the eight 

 winter months, the rain falls very heavily here, and that, in 

 consequence, the watershed is very great. I am not aware 

 of this fact, but, on the contrary, during the four summer 

 months, when they say there is no watershed, we have some- 

 times, in consequence of the tropical heat, very sudden and 

 tropical rains. 



The increase of our Australian rivers during floods proba- 

 bly ranges from 50 to 100 times the volume of the ordinary 

 streams, and this accords with the opinion of Mr. Blandowski, 

 who possesses a great practical knowledge of the physical 

 peculiarities of the colony. The highest flood lines of the 

 Plenty, with the velocity assumed by the Committee of two- 

 and-a-half miles per hour, give seventy-five times the volume 

 of water that passes Yan Yean in December. 



How very difiicult is the result of their singular and elabo- 

 rate calculations. Having found according to these, aided by 

 the evaporation tables of Mr. Dempsey, that the ordinary 

 stream of the Plenty for eight months out of the twelve, is 

 ten times larger than it is in December, the highest flood 

 lines which they themselves could discover will only permit 

 of a remarkably small increase during floods, the greatest 

 being seven-and-a-half times the volume of the ordinary 

 stream. 



Such a result is so startling and incredible, that it is suffi- 

 cient, in my opinion, to warrant the rejection, by the Philo- 

 sophical Society, both of their calculations and Mr. Dempsey's 

 tables. 



Nor is this result more incredible than that the ordinary 



