Ill 



TABLE VIII. 



FOR DEDUCING THE RELATIVE HUMIDITY OF THE AIR FROM THE INDICATIONS, IN 

 ENGLISH MEASURES, OF THE DEW-POINT INSTRUMENTS. 



THE object of every Dew-Point instrument is to ascertain, by causing a part of the 

 apparatus to cool, the temperature at which the vapor contained in the air begins to 

 condense, in the shape of light dew, on the cooled portion of the instrument. It is 

 obvious that this is the temperature at which the atmosphere itself, if cooled likewise, 

 would be fully saturated by the amount of vapor present in the air at the time of the 

 observation. 



The temperature of the dew-point being known, all the hygrometrical conditions 

 of the air can be easily deduced from it. 



The Absolute Humidity, or the total amount of vapor in the atmosphere, is ex- 

 pressed by the number, in the Tables of Elastic Forces of Vapor, due to that temper- 

 ature. 



The Relative Humidity, or the degree of moisture, being the ratio of the quantity 

 of .vapor actually contained in the air to the quantity it could contain if fully saturated, 

 is expressed by the proportion 



Relative Humidity : 1 : : Force of Vapor at Dew-Point : Maximum Force of Vapor. 



Calling the 



Force of Vapor at the Temperature of the Dew-Point, /; 



Force of Vapor at the Temperature of the Air, F ; 

 then 



Relative Humidity = |. 



It is thus found by dividing the force of vapor due, in the Table of Elastic Forces, 

 to the temperature of the dew-point, by the maximum of the force of vapor due, in 

 the same table, to the temperature of the air at the time of the observation. F being 

 always greater than/, when the air is not saturated, the Relative Humidity is ex- 

 pressed by a fraction, which is termed the fraction of saturation. Making the point 

 of saturation = 100, in order to obtain this fraction in hundredths, we have 



Relative Humidity = f -^. 

 B 75 



