69 



The easiest mode of ascertaining the difference, 

 between the temperature of the air and an evaporat- 

 ing surface is by having two equally graduated ther- 

 mometers hanging in the hothouse, with the bulb of 

 one inclosed in a piece of thin muslin. Upon moisten- 

 ing this with water of the house's temperature — made 

 so by keeping it in the house — the number of degrees 

 this causes the mercury to sink will be the difference 

 between the air's temperature and that of an evapo- 

 rating surface. In the winter months the gardener 

 may be satisfied that the air has the desirable moist- 

 ness if that difference be not more than 6 degs. and 

 during the summer months 3 degs. This is much 

 less than the difference found by observation in a 

 tropical climate, but there the evaporation was pro- 

 moted by exposure to a free circulation of the air. 



Connected with this important portion of the prac- 

 tice of forcing is the following table, shewing the 

 number of grains of water at the temperatures 

 named, contained by a cubic foot of air saturated 

 with moisture. 



