The Weather 



47 



small vessel filled with water. The water is transferred by 

 the wicking to the cloth, where it is evaporated. The evap- 

 oration of the water from the cloth wrapping of the bulb 

 cools the mercury and lowers the mercury column in the 

 tube. The more rapid the rate of evaporation, the lower the 

 mercury in the tube will fall, and the greater will be the differ- 

 ence between the readings of the two thermometers. This 

 indicates that the air has much less water vapor than it can 

 contain. The slower the rate of evaporation, the less will be 

 the difference between the two thermometers, and the greater 

 will be the amount of vapor in the atmosphere. If the two 

 thermometers have the same readings, it is evident that the 

 air has all the moisture it can hold, or one hundred per cent, and 

 that evaporation cannot occur in this situation. 



FIG. 15. Relative humidity, per cent Fahrenheit scale. 



