70 HEAT. 



CAPACITY OF THE AIR FOR WA- 



What effect 



has heat upon TER INCREASED BY HEAT. - But, as the 



the quantity of 



vapor present weather grows warmer, the capacity of 

 the air for moisture is increased, so that at 

 100, it can contain twice as much as at 75, or 

 two cubic inches. On the other hand, as the weather 

 grows cooler, its capacity is diminished, so that at 50 

 it can hold scarcely more than half a cubic inch, and is 

 saturated by this comparatively small quantity. And, 

 in general, the capacity of the air for moisture is in- 

 creased by the elevation of its temperature. 



161. EFFECT OF WIND. Wind causes 

 evaporation to proceed more rapidly, not 



tity of vapor because the air in motion has any greater 



in the air ? r . , . 



capacity for moisture, but because new 

 portions of air are brought successively into contact 

 with the wet surface. As fast as one portion has im- 

 bibed a certain amount of moisture, another portion of 

 drier and more thirsty air takes its place. 



162. DEPOSITION OF MOISTURE. It fol- 



Explain the 



deposition of lows that air that is saturated, or, in other 

 words, has its full portion of moisture in 

 the form of vapor, must deposit a portion of it in the 

 form of water in cooling. Thus a cubic yard of sat- 

 urated air at 75, on being cooled down to 50, would 

 yield half a cubic inch of water, or half of the whole 

 quantity which it originally contained. If we sup- 

 pose the experiment to be performed in a glass vessel 

 where the eifect of cooling could be observed, we 

 should first see a mist or dew within the box, consist- 

 ing of the particles of water which the colder air can 



