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 

 w^ater. During the rainy season, the increase of the J ordan 

 and other streams is sufficient to raise the level ten or even 

 fifteen feet ; bu^t under the influence of a burning sun and a 

 dry atmosphere, the lake, in a 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. 



