Failure of the Yan Yean Reservoir. 149 
would 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. Iam 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 difficult 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 
limes 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 
