78 Diseases of Greenhouse Crops 



a difference of two degrees between the wet and the 

 dry bulb thermometer. Let us now refer to the 

 table under the column Dry hulh tliermorneter de- 

 grees^ where it is marked 62 degrees. Read across 

 Table 13 under the column depression of -wet bulb 

 thermometer in degrees until the column indicates a 

 difference of two degrees. In this case it is the 

 fourth column. The resultant figure, 89, will be 

 the relative humidity of the hothouse air. In other 

 words, a temperature of 62 degrees of the dry bulb 

 thermometer with a difference of two degrees of the 

 wet bulb thermometer will give a reading of 89 

 relative humidity of the air. In like manner, and 

 by referring to Table 13, which should be hung up 

 at a convenient place in the hothouse, the relative 

 humidity of the house may be obtained. There are 

 other simpler instruments by means of which the 

 reading of the relative humidity may be obtained 

 directly without the use of tables. The Mithoff 

 hygrometer, for instance, is a type of such an instru- 

 ment. However, they may readily get out of com- 

 mission, and thus become imreliable. 



Watering 



The importance of water for greenhouse plants 

 cannot be too emphatically stated. It has been 

 truly said that "he who does not know how to water 

 plants does not know how to grow them." Water 

 is essential to plant life. It has been intimated that 

 some crops evaporate from the leaves an amount 



