PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 511 



at a faster rate than tlie temperature, so tliat while the air at thirty-two 

 degrees Fahrenheit, can contain only the one hundrfid and sixtieth part of 

 its own weight of vapor, at one hundred and thirteen degrees it c^n con- 

 tain the twentieth part of its weight. Thus it appears that while the tem- 

 perature advances in an arithmetical scries, the capacity is accelerated in 

 a geometrical progression. A considerable increase of temperature, tliere- 

 fore will enable even a saturated atmosphere to receive a greatly augmen- 

 ted amount of vppor, and as it were, to swallow the clouds that may pass 

 into it, without any diminution of its own transparency. On the contrary, 

 when the temperature is di.minislied by the rapid union of two currents 

 of air saturated with yapor, the one being warm and the other cool, the 

 average temperature is so reduced that an excess of vapor exists, which 

 is incapable of sustaining itscH in the diminished capacity of the air, and 

 is necessarily precipitated in tlie form of rain. But when two currents of 

 air not fully saturated with vapor, are brought into contact, the precipita- 

 tion of moisture is slight, and mists only are produced. When the mists 

 thus precipitated are near the earth, they are called fogs, but when high 

 in the air they take the name of clouds. 



Saussure and Kratzenstein have investigated the nature of fogs and 

 mists. The vapor, in this condition, is found to consist of minute globules, 

 ujion which rings of prismatic colors were discovered, like those seen upon 

 suap -bubbles, but which are never observed upon drops of water. From 

 this discovery it was concluded tliat the globules are hollow, and filled 

 with air or gas. The size of these globules is greatest when the atmosphere 

 is very humid, and least when it is dry. 



Another fact must be noted. The temperature of the air diminishes with 

 the altitude, but the law of decrease is very irregular, beirg affected by 

 latitude, seasons, hours of the day, and a diversity of local circumstances. 

 It may, however, be assumed as a general rule, that a loss of beat occurs 

 to the extent of one degree, Fahrenheit, for every three hundred and forty- 

 three feet of elevation. But this is an average result, for the rate of de- 

 crease is very rapid near the earth, after which it proceeds more slowly, 

 and at the loftiest heights is again accelerated.* 



From this brief statement of the general principles governing the pro- 

 duction of fogs and clouds, it will be apparent that the higher portions of 

 the mountains of North Carolina must be refreshed by frequent rains. The 

 elevated balls, ever clad in mantles of cool air, stand as so many custom- 

 house officers to exact tribute from all the currents of air laden with vapor, 

 from the warm regions below, which attempt to sail over their summits. 

 These currents of air cannot but pause, when richly freighted, to divide 

 their treasures with the thirsty soils and mountain-springs. And even 

 when they are lightly burdened with vapor, and no rain can be condensed 

 from them, these passing currents often yield copious clouds of fog', cover- 

 ing the vegetation, by contact, with moisture, and promoting its more 

 vigorous growth. 



Nor are the mountain summits alone in the exactions they make upon 

 the moving atmosphere for its vapors. The mountain bases, all along tho 



• These brief statcmcDts are from Brocklcsby's Meteorology. 



