Moisture and Water Requirements 77 



Humidity Determination in the Greenhouse 



Perhaps the quickest and safest way of determin- 

 ing the relative humidity of the air in the hothouse 

 is by means of a sling psychrometer (fig. 11.). 

 This instrument is very simple in design. It con- 

 sists of a wet and dry bulb thermometer attached 

 to a wooden or metal support. The handle arrange- 

 ment permits the instrument to be whirled in the 

 air while taking the reading. The wet bulb ther- 

 mometer is covered with muslin and is thoroughly 

 moistened by being plunged into a cup of water 

 which should be of the same temperature as that of 

 the air of the hothouse. 



Method of Determining Humidity 



After wetting the muslin of the wet bulb ther- 

 mometer, the instrument is whirled steadily for a few 

 seconds and the reading of the wet bulb thermome- 

 ter noted. This whirling is repeated several times, 

 until the reading of the wet bulb thermometer is 

 constant. At this stage, the difference of tempera- 

 ture between the wet and dry thermometers is re- 

 corded. After this difference has been obtained, we 

 turn to Table 13 to get the exact reading of the 

 relative humidity of the hothouse air. To make 

 this clear to the reader, let us take a specific exam- 

 ple. Suppose that the reading of the wet bulb ther- 

 mometer was 64 degrees, and that of the dry bulb 

 thermometer 62 degrees. This, then, will give us 



