BY T. L. BANCROFT. 217 



Reservoirs and permanent waterholes are seldom less than ten 

 feet deep, generally more, so that it would be impracticable, even 

 were it desirable, to grow weeds in them. Of course such plants 

 as Lemna and Azolla might be grown, provided the surface water 

 was not agitated much by wind. 



A number of glass cells each of the capacity of one gallon of 

 water were arranged some with and some without water-weeds ; 

 one series was placed outside in the sun, another series in the 

 shade and under cover. Duckweed (Lemna), Azolla, and the 

 Blue water-lily (Nyinj)h(m gigantea. Hook.) were the plants used. 

 From these experiments it was seen that evaporation was neither 

 retarded nor hastened. 



Oil floating in a thin layer on the surface, however, hindered 

 evaporation very considerably. 



Artificial dams and reservoirs of any kind should of course be 

 made as deep as possible with the object of preventing the water 

 becoming hot, and as presenting the least surface for evaporation. 



Brisbane, May 9th, 1893. 



