-1859] WHITINGS OF JOSEPH HENRY. 235 



necessary to produce complete saturation, the relative humid- 

 ity is marked 25. To find then the relative humidity at any 

 time, we seek from the tables the tension of vapor due to the 

 temperature of the air, and again its tension due to that tem- 

 perature to which it must next be cooled down in order to pro- 

 duce precipitation, or full saturation, which temperature as 

 we have seen is that of the dew-point. We then say, as the 

 tension of the first temperature is to 100, so is the tension of the 

 other temperature to the percentage of saturation. In this way 

 comparative tables of relative humidity for different places 

 are calculated from actual observation. 



Instead of employing the method of the dew-point for as- 

 certaining the quantity of vapor in the atmosphere, a process 

 which is attended with some difficulty, particularly in cold 

 weather, since in this case it is not easy to reduce the tem- 

 perature of the water within the tumbler except by a freez- 

 ing mixture sufficiently low to produce the deposition of dew, 

 another process has been employed, called that of the wet 

 and dry bulb thermometer. 



In this process we note the temperature of the air by an 

 ordinary thermometer, and again we observe the tempera- 

 ture to which in the same place a thermometer, whose 

 bulb is covered with wet muslin, descends. If the air is 

 perfectly saturated with moisture the two thermometers will 

 indicate the same degree ; but if the temperature is above 

 that due to the elastic force of the actual amount of vapor 

 in the air the evaporation from the moist bulb will cause it 

 to descend, by the absorption of heat, a certain number of 

 degrees below that indicated by the naked bulb. 



M. Regnault has compared by direct experiment, the in- 

 dications of the wet and dry bulb thermometer, with the 

 actual amount of vapor contained in air at different tem- 

 peratures and at different degrees of saturation, according 

 to the method previously explained, and has in this way 

 formed a series of tables by which the dew-point, the ten- 

 sion of the vapor, and the weight in a cubic foot can be ascer- 

 tained. In order however that these indications may be 

 relied upon, it is necessary that the observations be made 

 with care, since the evaporation from the wet bulb will very 



