Finding the Dew-point. 



131 



wet -bulb and then whirl or fan the instrument, 

 when the temperature will fall. Continue the ven- 

 tilation until the wet -bulb thermometer ceases to fall, 

 when the two thermometers should be read. Subtract 

 the reading of the wet -bulb thermometer from that of 

 the dry. Find this difference in the column at the 

 left of the table. The dew-point will then be 

 found at the intersection of the line opposite this 

 difference and the column which is headed by the 

 number nearest the air temperature (dry -bulb 

 reading). Examples are given below: 



" Dry -bulb thermometer 55 



Wet-bulb thermometer 44 



Difference 11 



Dew-point from table 30 " 



The dew-point is the temperature at which the deposition of dew be- 

 gins. Frost is formed when the dew-point and freezing-point coincide 

 (that is, at 32). The nearer the dew-point approaches the freezing-point 

 nt nightfall, the greater is the danger ot frost during the niglit. When 

 the dew-point is 10 above freezing-point at nightfall (42), there is little 

 danger of frost; but when it is less than this, frost may be expected. 



w DEW -POINT TABLE. 



