510 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Clayton, Georgia, is located not far from Rabun Gap — a low depression 

 in tlie Blue Ridge. Tliis depression consists of some swampy lands in 

 which the head watcr-< of the Little Tennessee and the Savannah Rivers 

 take their rise. Tiie mountains on oflch side of this gap rise to the height ~ 

 of fifteen hundred [eet. On the morning after my arrival at this town, my 

 travelling companion awoke me to call my attention to a wonder. 



The sun was just rising. On looking out at the window toward the 

 north, I beheld a vast volume of fog, filling Rabun Gap from base to sum- 

 mit, and occasionally extending even above the highest parts of the moun- 

 tains. It was as white as snow, and resembled a vast deluge of cotton as 

 it falls loosely from tlie gin. In front of the main gap, and between it and 

 the town, there stacds a small mountain, detac|ied from the principal range, 

 with a gap upon each side. The fog as it rolled through the main gap, 

 deflected into the smaller gap, to the east of the little mountain. 



On viewing it for a few minutes, I was soon startled by noticing, that 

 though the whole immense volume of the fog was rolling forward at quite 

 an observable rate of speed, yet it never passed much beyond the southern 

 side of the little mountain. Onward it came with a sufficient force, and 

 bulk sufficient to overwhelm in jts darkness, the whole southern side of the 

 Blue JRidge. But beyond the line named it could never pass. A barrier 

 existed there, in the different conditions of the atmosphere, which at once 

 dissolved the fug, and left the air beyond as transparent as ever. Once in 

 a while a small portion of the fog would whirl forward, a few hundred 

 feet beyond the main mass, like a bold leader in front of an army, as if to 

 encourage the forces behind to move onward with greater daring. But all 

 was in vain, as leader and follower were quickly involved in a similar fate. 

 The law which controlled the movements of the fog, said to it emphatically, 

 "Hitherto shalt thou come, but no further." 



Turning to my friend who had patiently watched me while I was absorbed 

 in contemplating this wonderful phenomenon, I asked him if it had ever 

 occurred before. " Yes sir," he answered, " it occurs every clear morning 

 from spring to fall. Beginning to roll through a little before, the sun 

 appears above the horizon, it continues till eight o'clock sometimes, and as 

 late as ten at others; and this it repeats every clear morning, and has 

 repeated, doubtless ever since the dry land appeared, and the mountains 

 and the rivers were formed." 



Again I turned to view the fog, and found it coming on to its fate, as 

 regardless of consequences apparently, as we poor thoughtless mortals 

 are when treading upon the very verge of destruction. 



As explanatory of the phenomena at Clayton, and of the production of 

 clouds and rain in the mountains, a few general principles in natural phi- 

 losophy must be stated: 



At all temperatures, moisture exists in the atmosphere in an invisible 

 state. It sustains itself there in the intervals that exist between the par- 

 ticles of air. These intervals are either partially or wholly filled with 

 vapor constantly arising from the earth. When they are wholly filled with 

 vapor, the atn)osphere is said to be saturated. An increase of temperature 

 by dilating the air, increases its capacity for moisture ; while a diminution 

 of temperature is followed by contrary elTects. But the capacity increases 



