12 Aqueous Vapor. — JDeio Point. — Absolute Humidity. 



52. Aqueous Vapor. The proportion of watery vapor in the air 

 has a most important influence upon tree gi-owth, and where the 

 amount is small, their cultivation becomes difficult or impossible. 



53. Water, when exposed, will slowly evaporate, the rate being 

 greater at a high temperature and in a dry air. It is still greater 

 when there is a wind passing over the surface, carrying off the va- 

 por and bringing dry air in its place. 



54. Aqueous vapor is always present in the air, although it may 

 be imperceptible to our senses. There is, however, a limit, above 

 which the excess becomes visible as fog or cloud, and falls as dew or 

 rain. If the temperature falls below the freezing point, the dew be- 

 comes Jwar frost, and the rain becomes snow. 



55. The degree of temperature at which condensation begins is 

 called the dew point. It may be ascertained by cooling down water 

 in a bright and thin metallic cup until dew begins to form on the 

 outside. 



56. The humidity of the atmosphere is usually ascertained by the 

 psychrometer, which consists of two similar thermometers set a few 

 inches apart, one of them having the bulb covered with white mus- 

 lin cloth, which is wet before an observation is taken. The wet 

 bulb is gently fanned till the temperature goes down to a stationary 

 point, and then both thermometers are read. By the aid of tables 

 that have been computed for this purpose, we may very easily ob- 

 tain from the temperature of the dry bulb instrument, and the dif- 

 ference between that and the wet bulb, two separate statements con- 

 cerning the moisture present in the air, viz.: the ahsolute and the 

 relative humidity. 



57. The Absolute Humiditij is the elastic force or tension of the 

 vapor, as would be shown in its raising a column of mercury in a 

 guage, and is usually given in decimal parts of an English inch. 

 With a given difference between wet and dry bulb thermometers, 

 it increases with the temperature, being greatest when the weather 

 is warmest, as shown by Diagram 1. We see, for example, that at 

 90° it is 1.3 inches, the difference between thermometers being 10°, 

 while it is but 0.3 at 55°, and but a little over 0.1 at 40°. The 

 rate gains rapidly at high temperatures, and above the boiling point 

 it becomes the power of steam. 



58. As observed in a very warm atmosphere, there may be an 

 abundance of moisture present in the air and no rain. At a fixed 



