132 Failure of the Yan Yean Reservoir. 
- Being anxious to ascertain the rate of evaporation over a 
large surface, fully exposed to the influences of the weather, 
I lately selected a sheet of water of about 300 yards long, 
with an average depth of eighteen inches, and width of four- 
teen feet, and, by fixing a mark in a particular part of the 
bank, I made careful measurements several times during 
fourteen days, and found that the amount lost exactly equalled 
six inches in that time, which gives five lines per day, and, 
for the three summer months, three feet two inches. The 
weather throughout was cool, excepting one day, and the 
winds southerly and easterly, so that this may be regarded 
as the lowest rate at this season. By computing the evapo- 
ration of the other months according to their mean tempera- 
ture, this rate would give nine feet for the twelve months. 
In further illustration of this subject, I may mention that 
Mr. Laidlaw has computed the evaporation in Calcutta at 
fifteen feet, and he also found that it averaged nearly three- 
fourths of an inch a day, between the Cape of Good Hope 
and Calcutta, and between 10° and 20° in the Bay of 
Bengal, he found it to exceed one inch daily, or at the rate 
of thirty feet in the year. 
Dr. Milner mentions that there are many lakes in the steppes 
of Northern Asia which have no natural outlet. Some of 
them are many miles in circumference, and have a depth of 
six and seven feet, from the winter rain, but are entirely 
evaporated during the summer months. 
And, according to Irby and Mangles, who describe the 
effects of evaporation in the Dead Sea, it must be very rapid 
indeed, notwithstanding the strong saline impregnation of the 
water. During the rainy season, the increase of the Jordan 
and other streams is sufficient to raise the level ten or-even 
fifteen feet; but under the influence of a burning sun and a 
dry atmosphere, the lake, ina few months, resumes its former 
level. 
It remains to consider the loss from absorption. 
At the last meeting of the Society I expressed great fears 
that a serious loss might be sustained in the reservoir from 
this cause; but I have since seen the evidence which Mr. 
Hodgkinson gave on the subject before the Select Committee, 
which I consider perfectly satisfactory. 
To determine the exact amount of water, that will be 
available for the use of the city, I have now to deduct the 
loss from evaporation. si 
