20 Measurement of Evaporation and Percolation. 



level with a line, N, M, and left. When next observed, the water 

 is drawn down to A, and the scale will show how much has been 

 wasted by evaporation. More water is added, and the scale is 

 again set. 



81. By a simpler plan, known as Piche's evaporator, a plain grad- 

 uated glass tube, with its lower end open and ground flat, is filled 

 with water. A disc of paper, first wet, is applied. It is then turned, 

 with the closed end up, and the paper is found to adhere by atmos- 

 pheric pressure. It continues wet, and by evaporation the Avater 

 Avastes away, the amount of loss being shown on the scale. 



82. An open cylindrical dish of Avater may be used, the depth 

 being measured at the beginning and end of the observation, and 

 the loss supplied from time to time. The measurement may be 

 made vertically by a scale, or by volume in a graduated meas- 

 uring cup, or by Aveighing. It is necessary to cover with a wire 

 screen, to prevent birds from bathing in it. 



83. By an instrument shown in the annexed cut, the evapora- 



tion from soils, either 

 naked or covered with 

 grass, litter, or herb- 

 age, or by small grow- 

 ing trees, may be meas- 

 ured. The zinc-lined 

 box, A, is connected 

 with a reservoir of 

 Avater, C, through a 

 valve, E, so as to keep 

 Avet up to a certain 



8. Instrument for Measuring the Evaporation from level. The Avater may 



Soils< be drawn off from the 



faucet. The amount supplied from time to time shows the rate of 

 evaporation. 



84. The percolation of Avater through soils is measured by a 

 Lysimeter. It consists of a vessel of known area at the surface, A, 

 set in the ground, and the surface either clearer covered Avith litter, 

 herbage, etc. The rains that fall on the surface filter down to the 

 tube, C, and are measured in the receiver, D. 



