July 30. 191 7 Evaporation from Water and River-Bed Materials 235 



Table XII. — Relation between evaporation from United States Geological Survey 

 standard floating pan and land pans 



"Diameter, 12 feet, and 3 feet deep, set in ground 2.7s feet. 



b 3 feet square by 3 feet deep, set in ground 2.75 feet. 



<: United States Geological Survey standard floating pan, on near-by lake, 3 feet square by i.s feet deep. 



50.63 

 d Total evaporation from tank 3 expressed as a percentage of the total from No. 9=— — Xioo=ii5.3. 



Figure 6 shows the relative temperatures of air (in instrument shelter), 

 surface of water in tank 3, surface of water in tank 7, surface of water in 

 tank 9, and the lake. Observations were taken both of the water in the 

 tank at the lake and the lake water itself. The means were invariably 

 the same, but occasionally the tank water would become i degree colder 

 at night and i degree warmer during the day. No greater difference than 

 I degree occurred. The difference is evidently due to influence of the 

 radiation and concentration of the metal float tubes and the metal of 

 the tank itself. The effect of heat storage is shown by the mean temper- 

 ature curve; also the greater influence of the warm days over the cool 



