But a conſtant iſſue of air, only varying in its force as the weather is dryer or damper, will require a new hypotheſis. There is another blowing cave in the Cumberland mountain, about a mile from where it croſſes the Carolina line. All we know of this is, that it is not conſtant, and that a fountain of water iſſues from it.
The Natural Bridge, the moſt ſublime of nature's works, though not comprehended under the preſent head, muſt not be pretermitted. It is on the aſcent of a hill, which ſeems to have been cloven through its length by ſome great convulſion. The fiſſure, juſt at the bridge, is, by ſome admeaſurements, 270 feet deep, by others only 205. It is about 45 feet wide at the bottom, and 90 feet at the top; this of courſe determines the length of the bridge, and its height from the water. Its breadth in the middle, is about 60 feet, but more at the ends, and the thickneſs of the maſs, at the ſummit of the arch, about 40 feet. A part of this thickneſs is conſtituted by a coat of earth, which gives growth to many large trees. The reſidue, with the hill on both ſides, is one ſolid rock of lime-ſtone. The arch approaches the ſemi-elliptical form; but the larger axis of the ellipſis, which would be the chord of the arch, is many times longer than the tranſverſe. Though the ſides of this bridge are provided in ſome parts with a parapet of fixed rocks, yet few men have the reſolution to walk to them, and look over into the abyſs. You involuntarily fall upon your hands and feet, creep to the parapet, and peep over it. Looking down from this height about a minute, gave me a violent head-ache. If the view from the top be painful and intolerable, that
