234 WRITINGS OF JOSEPH HENRY. [1855- 



the air is just saturated the dew makes its appearance. 

 Hence when the sides of the vessel are very thin the tem- 

 perature noted by the thermometer within gives that of the 

 dew-point without, and if we inspect the table for this tem- 

 perature we find at once the corresponding tension and 

 weight of vapor in that portion of the atmosphere in which 

 the experiment was made. 



It is not however upon the actual amount of vapor which 

 the air contains at a given time or place that its humidity 

 depends ; but upon its greater or less degree of saturation. 

 That air is said to be dry in which evaporation takes place 

 rapidly from a surface of water or moistened substance. In 

 an atmosphere entirely saturated with vapor, that is in one 

 which is filled with as much vapor as the space which it 

 occupies can contain, the vapor already in the air by its elas- 

 tic force presses on the surface of the moist body and neu- 

 tralizes the repulsive action of the water; if however the 

 temperature be raised, the elastic force will be increased, 

 and a new portion will be forced into the same space; the 

 farther therefore the condition of any portion of air is from 

 saturation the more rapid will be the evaporation from the 

 moist bodies which it surrounds. 



For example, a portion of saturated air at a temperature 

 of 102° would contain vapor of an elastic force equal to a 

 pressure of 2 inches of mercury. (See table A, p. 217.) If 

 the same air however contained vapor of only the elastic 

 force of 59°, (that is if the dew-point were at 59°,) the elastic 

 force would be half an inch, and consequently there would 

 be a force unbalanced by the pressure of vapor equal to the 

 pressure of a column of 1| inches of mercury. The dryness 

 therefore of the air is estimated by the difference of the 

 elastic force of the vapor due to the temperature of the air, 

 and of the elastic force due to the tension of the dew-point. 



In meteorological works generally, a portion of the atmos- 

 phere containing vapor equal in tension to that of the tem- 

 perature of the air is said to be fully saturated, and its 

 humidity is marked 100; but if the elastic force of the air 

 as determined by the dew-point is only one-fourth of that 



