-1859] 



WRITINGS OF JOSEPH HENRY. 



237 



average temperature of the water during the day, and for 

 this purpose a bulb of a thermometer is placed just below 

 the surface of the liquid. In ascertaining the evaporation 

 from different kinds of soil, a number of boxes of the dimen- 

 sions above described, should be filled with different sam- 

 ples, supplied with a measured quantity of water, weighed 

 from day to day, and the loss (which will give the evapor- 

 ating Ccapacity) accurately noted. To ascertain the amount 

 of evaporation from the actual surface of the earth in the 

 course of the year, the loss should be daily determined from 

 a new portion of earth taken from the surface in its actual 

 condition. 



The annual amount of evaporation from a given surface 

 of water in the interior of the country is greater than that 

 of the rain which falls on the same surface, but the amount 

 of evaporation from the surface of ground is generally less, 

 particularly in mountainous districts. 



The evaporation does not depend upon the position of the 

 evaporating surface since a piece of moist paper pasted on 

 a pane of glass loses the same amount of water in the same 

 time, whether it be held horizontally or vertically. It does 

 however depend very much upon the nature of the surface; 

 for example, less must be given off in a given time from a 

 surface of salt water than from a surface of fresh water; and 

 also from the cohesion with which water adheres to solids, 

 a less amount of vapor is produced in a given time from a 

 given surface of moist earth than from water, as is shown 

 by the following table, deduced from observations made by 

 M. Gasparin, in France, at temperatures from 73° to 75°' 

 F., during the time specified : 



Dates. 



1st day of August 



2d do. 



3d do. 



4th do. 



5th do. 



6th do. 



7th do. 



Evaporation 

 from water. 



Inch. 

 0-575 

 0534 

 0-448 

 0-468 

 0-456 

 0-429 

 0-367 



Evaporation 

 from earth. 



Inch. 

 0-160 

 098 

 0070 

 0-051 

 0-051 

 0-047 

 0-051 



