48 Introduction to the Study of Science 



The dry-bulb thermometer gives the air temperature. The 

 wet-bulb thermometer gives what is sometimes called the sensible 

 temperature, that is, the temperature which we experience or 

 feel. 



Exercise. To obtain the relative humidity in percentage we take 

 the readings of the two thermometers, find the difference between 

 them, and then consult a table that gives the relative humidity in 

 per cent. Suppose the dry-thermometer reading is 80 F. and the 

 wet 70, giving a difference of 10. In the table, Fig. 15, begin with 

 the dry-thermometer temperature of 80 and follow the row of figures 

 across the table until you reach the figure in the column headed by 

 the difference 10. This figure gives the relative humidity in per cent, 

 or 61 per cent. 



1. The reading of the wet-bulb thermometer is 45 and that of the 

 dry is 69. What is the relative humidity ? 2. The wet thermometer 

 is at 78, and the dry at 80. What is the relative humidity ? 



27. Temperature and condensation. The influence of 

 temperature upon condensation is to be considered more fully. 

 Suppose a mass of air saturated at 68 F. is cooled to 59 F. 

 without other change. What will occur ? Air at 59 F. cannot 

 hold in the form of vapor as much moisture as it can at 68 F. 

 The result is that with the fall of the temperature below 68 F., 

 vapor begins to condense upon dust motes, of which there is 

 an incalculable number in the atmosphere, and in this way 

 forms cloud or fog. When deposited upon grass and other like 

 objects, it forms dew. In other words when the temperature 

 falls from 68 F. to 59 F., 0.12 of an ounce of water vapor 

 per cubic yard is condensed or changed into liquid water 

 (Fig. 13). 



The dew point. Does the cooling of the air below the point 

 at which condensation begins increase or decrease the absolute 

 humidity? If the humidity of a mass of air remains at .35 

 of an ounce per cubic yard, and the temperature at any point 

 above 59 F., the water vapor will not condense. But as soon 

 as the temperature reaches 59 F., the point at which saturation 

 is 100 per cent, and continues to fall below this point, condensa- 



