Failure of the Yan Yean Reservoir. 



125 



found, with a mean rain of 33.55 inches for three years, that 

 tire evaporation from the ground equalled 25 -14 inches; 

 therefore 8 -41 inches, or one-fourth of the rain, may be 

 presumed as the amount available for springs and rivers in 

 England. 



I am not aware that any experiments have yet been con- 

 ducted in this colony to determine the proportion of the rain 

 that is evaporated. 



The physical character of the country, however, shows 

 that there are comparatively few rivers, and that these are 

 very scantily supplied with water, and that floods are not of 

 frequent or regular occurrence. The absence of high moun- 

 tains, and the arid and desert condition of the interior^ while 

 they greatly diminish the rainfall, contribute to render the 

 atmosphere peculiarly dry, and evaporation very rapid. 



In the summer months many rivers and creeks are dried 

 up, and the ground becomes so parched that it is capable of 

 quickly absorbing a large quantity of rain, so that it is rare 

 to find rivers much increased during this season, and the 

 watershed is very trifling indeed. 



I think, therefore, that with nearly the same amount of 

 rain as in England, and with a much smaller proportion of 

 rivers and fewer floods, there must be a much larger propor- 

 tion of rain evaporated from the surface. 



I may thus, on very strong grounds, assume that the 

 watershed in this colony is one half less in proportion than 

 in England, and therefore amounts very nearly to Dr. Thom- 

 son's estimate, which is one-ninth of the rain. 



But it is more satisfactory, and certainly more correct, to 

 deduce the watershed of this colony, from that of England, 

 by making adequate allowance for the difference in the force 

 of evaporation due to our higher mean temperature ; and, as 

 it is admitted by scientific men here, that the evaporation 

 from the surface of water is nearly double that of England, 

 it is strictly correct to assume that the proportion of rain 

 evaporated here will also be nearly dovible. 



Thus, I think, no valid objections can be offered to my 

 assuming four and a-half inches instead of eight and a-half, 

 as the best approximation that can be made of the watershed 

 of this colony, until the mean discharge of the different rivers 

 is ascertained by actual measurement. 



In mountainous districts the watershed is greater in pro- 

 portion than in the low country, and the absorption and 

 evaporation less ; and therefore it might be thought that Dr. 



