hill, is of about loo feet diameter, and emits con- ftantly a current of air, of fuch force, as to keep the weeds proftrate to tiie dif- tance of twenty yards before it. This current is ftrongeft in dry, frofty weather, and in long fpells of rain weakeft. Re- gular infpirations and expirations of air, by caverns and fifllires, have been proba- bly enough accounted for, by fuppofing them combined with intermitting foun- tains; as they muft of courfe inhale air while their refervoirs are emptying them- felves, and again emit it while they are filling. But a conftant iflue of air, only varying in its force as the weather is drier or damper, will require a new hypothefis.
There is another blowing cave in the Cumberland mountain, about a mile from w^here it croffes the Carolina line. All we know of this is, that it is not conftant, and that a fountain of water ilTues from it.
The Natural Bridge^ the moft fublime of nature's works, though not compre- hended under the prefent head, muft not be pretermitted. It is on the afcent of a hill, which feems to have been cloven through its length by fome great convul- fion. Tne fiflure, jufl at the bridge, is, by fome admeafurements, 270 feet deep, by others only 205. It is about 45 feet wide at the bottom, and 90 feet at the tcpi this of courfe 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 thicknefs of the mafs, at the fummit of the arch, about 40 feet. A part of this thicknefs is conftituted by a cx)at of earth, which gives growth to many large trees. The refidue, with the hill on both fides, is one folid rock of lime-ftone.—The arch approaches the femi-elliptical form; but the larger axis of the ellipfis, which would be the cord of the arch, is many times longer than the tranfverfe. Though the fides of this bridge are provided in fome parts with a parapet of fixed rocks, yet few men have refolution to walk to them, and look over into the abyfs. You invo- luntarily fall on your hands and feet, creep to the parapet, and peep over it. Look- ^^Hb^^ih^ hill, which feems to have been cloven through its length by fome great