Tlte Ming Psychrowieter. 129 



and the temperature of complete saturation or dew- 

 point, by measuring the reduction of temperature by 

 evaporation . 



"A sling psychrometer can easily be made, as fol- 

 lows : For the frame, take a board eighteen inches 

 long, two inches wide, and one -half inch thick, with 

 a hole bored in one end to hang the apparatus on a 

 nail when not in use. Get two all -glass thermom- 

 eters with cylindrical bulbs, and the degrees Fahren- 

 heit engraved on the stem. Cover the bulb of one 

 thermometer with a thin piece of cotton cloth, 

 fastening it securely by a thread. When this cloth 

 covering is wet with water and exposed to evapo- 

 ration in the air, it constitutes the 'wet -bulb ther- 

 mometer ' ; the other thermometer has no covering 

 on its bulb, is not wet at any time, and constitutes 

 the 'dry -bulb thermometer'. 



"Securely lash the thermometers on opposite edges 

 of the narrow board, leaving the graduations on 

 them plainly in sight, and the bulbs extending a 

 short distance below the end of the board. To 

 use the instrument, wet the cloth -covered bulb with 

 water, leaving the other bulb dry, and then swing 

 the apparatus freely through the air for three to 

 five minutes, or until the wet -bulb thermometer 

 ceases to fall in temperature, and then read the 

 temperature of each thermometer. Unless the air 

 is saturated with moisture the wet -bulb will always 

 show a lower temperature than the dry -bulb. Sub- 

 tract the degrees of wet -bulb from those of the 

 dry -bulb, and the remainder will show the degrees 

 j 



