III. 



TABLE 



giving at sight the relative humidity deduced from the indica- 

 tions of the dew point instruments. 



By M. T. Haeghens. ' 



This table, which has been published in the Annuaire Meteor ologique de France 

 for 1850, page 86, and following, has been calculated by Mr. Haeghens, using 

 Regnault's Tables of Elastic Forces of Vapor. It gives directly the relative humidity^ 

 when the hygrometrical observations have been made by means of dew point instru- 

 ments like those of Daniell, Regnault, Bache, and others. 



These hygrometers are destined to find out the temperature of the dew pointy that 

 is the temperature to which it would be necessary to lower the temperature of the 

 air, in order that this air be completely saturated by the aqueous vapor which it con- 

 tained at the time of the observation. 



The force of vapor contained in the air, or its absolute humidity, is thus the maxi- 

 mum of force of vapor which corresponds to the temperature of the dew point ; it is 

 given directly in the Table I. of the Elastic Forces of Vapor, by Regnault. 



The ratio of that maximum of force of vapor at the temperature of the dew point 

 to the force of vapor which corresponds, in the same table, to the temperature of the 

 surrounding air at the time of the observation, is the relative humidity. This ratio is 

 given in hundredths in the following table, which relieves the observer of the trouble 

 of calculating it. 



Let t =■ temperature of the air surrounding the instrument. 

 t' = temperature of the dew point. 

 t — t' ■= the difference between these two temperatures. 



The first column, on the left, contains the temperature of the air t, in centigrade 

 degrees. The following ones, headed with the differences, t — t', between the 

 temperatures of the air and of the dew point, give the relative humidity correspond- 

 ing to the two elements. 



Temp, of the Air = t. Dew point = t>. Differeiice / — t'. Relative Humidity. 



Example: 10°.0 4°.4 5°.6 68 



Should the temperature of the air t', or the difference t — <', fall between the 

 numbers found in the columns, it is obvious, by glancing at the table, that an inter- 

 polation at sight will always be easy. 



B 30 



