148 Failure of the Yan Yean Reservoir. 
compared with Mount Disappointment, and these are covered 
with snow during the winter months. But it has yet to be 
proved that the Yarra undergoes so remarkable an increase 
in volume for a period of eight months in the year. This 
river has an average depth of thirty feet, for a distance of two 
miles above Prince’s Bridge, and I need scarcely say that it 
only overflows its banks in floods, which may not occur once 
in two years. Its level, in the beginning of December, is 
not more than two or three feet below the average of the 
winter months, or below the level of many portions of the 
banks ; with any reasonable increase in the velocity, therefore, 
how is it possible for the Yarra, with an increase of only 
one-tenth in its depth or sectional measurement, to carry ten 
times the volume of water for eight months? 
We have only then to compare, in the drawings furnished 
by the Committee, the sectional area of the Plenty, at one 
or more points, in order to see how impossible it is to believe 
that the river, during eight months of the year, could contain 
ten times the volume of water that it does in December. 
The drawings show that the stream occupies in January 
one-half of the sectional measurement, that is, one-half of 
the depth where the banks are perpendicular. 
Now, as above stated, it may be regarded as an axiom, 
that when a river has defined banks these indicate its 
ordinary limits, which it only exceeds in time of floods. 
Thus, without actual measurements for the winter months, 
important information as to the volume of water may be 
gathered from those that are resident on the spot. Dr. 
M‘Kenna and myself put the question to Mr. Bear, who 
has long resided on his own property at Yan Yean, if a 
measurement of the Plenty, on the 12th of December, 
would give a fair average for the year. He replied, that 
he thought it would. Now, the meteorological tables show 
that November is our wettest month, and Mr. Blackburn’s 
measurement of 2,700 gallons per minute was taken in 
December; therefore, it is not unreasonable to suppose, 
that Mr. Bear’s opinion may be very nearly correct. 
But let us examine the section of the river, at the entrance 
of the aqueduct, not very far distant from Mr. Bear’s house. 
With a discharge in January of 2,537 gallons per minute, 
and a velocity of half a mile per hour, the section could 
rot contain more than twice the present volume, with the 
same velocity, without spreading widely over its right bank, 
which is nearly level. With double the volume the velocity 
