120 Failure of the Yan Yean Reservoir. 
situation where it could not fail to be considerably augmented 
by the rainfall of fifteen hours. The measurement of the 
eastern arm, which was obtained the following day, could not 
give any certain result, as it was taken above the junction of 
three small tributaries. In one point of view, however, it is 
very important, as the object of the measurement was to 
illustrate the constancy of the supply. All the other tribu- 
taries of the Plenty have been known to fail on several 
occasions. 
The measurement of the main eastern branch, therefore, 
shows an approximation to the amount of water supply 
that would be available above the swamps in a severe 
drought, but it is only an approximation, as it too may 
also fail; there is certainly no guarantee in the height of 
Mount Disappointment, which is only 1,500 feet, to warrant 
a more favourable opinion. 
All the tributaries of the Plenty equally depend on the 
rainfall of the mountain ranges, and it is simply because the 
main eastern branch rises from the highest point, where 
there is most rain and least evaporation, that it holds out the 
longest when the ordinary supply of rain is cut off. 
I come now to mention the measurements which were made 
by the Committee who were appointed to investigate this 
subject at the last meeting of the society. 
The river, at its junction with the aqueduct, with a velo- 
city of half a mile an hour, gave a discharge of 2,537 gal- 
lons per minute, or an equivalent to three feet four inches 
in the reservoir. The eastern arm above the swamps 
yielded 4,450 gallons per minute, or an equivalent to five 
feet eleven inches. If we take Mr. Hodgkinson’s measure- 
ment of the western arm, where it issues from the rocks, 
namely, 1,180 gallons per minute, and add to it the Com- 
mittee’s measurement of the eastern arm, we shall have 
5,630 gallons as the discharge of all the streams above the 
swamps; and if we take into consideration that one week 
before their visit to Yan Yean there were nearly three days 
of heavy rain, which is an unprecedented occurrence in 
January, the 630 gallons may be regarded as due to this 
source, and the balance of 5,000 gallons per minute will 
exactly correspond with Mr. Blackburn’s estimate. 
From these measurements, it would also appear that the 
eastern arm bears the proportion to the western arm of 4,450 
to 1,180, or very nearly four to one. By this estimate we 
shall find that the discharge of the western arm was 1,340 
