THE WATER VAPOR OF THE AIR 217 



in motion, the moisture will be swept away as fast as it evapo- 

 rates from the wet bulb, and the instrument will show a 

 lower humidity than it would in still air with the same amount 

 of moisture. The relative humidity table, Table XI, page 

 218, assumes that the air about the thermometers is moving 

 12 ft. per second, or 8 miles per hour, which is a comfortable 

 summer breeze. To secure this movement of air the sling 

 psychrometer, or other whirling apparatus, is used when ac- 

 curate observations are desired. 



248. Use of the Relative Humidity Table. A glance at the 

 table shows how to use it in finding the humidity. 



1. The temperature of the air, which is obtained from the 

 dry thermometer, appears in the left-hand column of the 

 table. 



2. The cooling caused by evaporation, which is the differ- - 

 ence between the dry and wet thermometer readings, is 

 found in the horizontal line across the top of the table. 



3. The relative humidity is expressed in per cent, and is 

 found in the table where the horizontal line from the dry 

 temperature crosses the vertical column that is headed by the 

 proper " difference" (dry minus wet) in the top line. 



For example: If the dry thermometer reads 70 and the wet 65, 

 their difference is 5. Find 70 in the left-hand column; follow that 

 line across the page to the column headed 5. In that 5-column, 

 on the 70 -line, is the number 77. This 77 is the relative humidity, 

 and means that the moisture present is 77 per cent, of what could 

 exist as invisible vapor at a temperature of 70. 



PROBLEMS 



If the dry bulb reads 50 and the wet bulb 44 . 0, the rel. humid, is 62. 

 If the dry bulb reads 90 and the wet bulb 75.0, the rel. humid, is ? 

 If the dry bulb reads 80 and the wet bulb 68 . 0, the rel. humid, is ? 

 If the dry bulb reads 35 and the wet bulb 32 . 0, the rel. humid, is ? 

 If the dry bulb reads 40 and the wet bulb 35 . 0, the rel. humid, is ? 

 If the dry bulb reads 45 and the wet bulb 42 . 5, the rel. humid, is 82.5 

 If the dry bulb reads 73 and the wet bulb 60. 8, the rel. humid, is 49. 

 If the dry bulb reads 40 and the wet bulb 38 . 8, the rel. humid, is ? 

 If the dry bulb reads 50 and the wet bulb 46 . 5, the rel. humid, is ? 



