perpendicular height, the ascent of which, on one fide, is so steep, that you may pitch a biscuit from its fummit into the river which washes its base. The entrance of the cave is, in this fide, about two thirds of the way up. It extends into the earth about 300 feet, branching into fubordinate caverns, sometimes afcending a little, but more generally descending, and at length terminates, in two different places, at bafons of water of unknown extent, and which I fhould judge to be nearly on a level with the water of the river; however, I do not think they are formed by refluent water from that, because they are never turbid; because they do not rise and fall in correfpondence with that in times of flood, or of drought; and because the water is always cool. It is probably one of the many refervoirs with which the interior parts of the earth are supposed to abound, and which yield fupplies to the fountains of water, diftinguished from others only by its being acceffible. The vault of this cave is of folid lime-ftone, from 20 to 40 or 50 feet high, through which water is continually percolatingThis, trickling down the fides of the cave, has incrufted them over in the form of elegant drapery; and dripping from the top of the vault generates on that, and on the base below, ftalactites of a conical form, fome of which have met, and formed maffive columns.

An

Another of thefe caves is near the North mountain, în the county of Frederick, on the lands of Mr. Zane. The entrance into this is on the top of an extenfive ridge. You defcend 30 or 40 feet, as into a well, from whence the cave then extends nearly horizontally, 400 feet into the earth, preferving a breadth of from 20 to 50 feet, and a height of from 5 to iz feet. After entering this cave a few feet, the mercury, which in the open air was at 50°. rofe to 57°. of Farenheit's thermometer, anfwering to i1°. of Reaumur's, and it continued at that to the remoteft parts of the cave. The uniform temperature of the cellars of the obfervatory of Paris, which are go feet deep, and of all fubterraneous cavities of any depth, where no chymical agents may be fuppofed to produce a factitious heat, has been found to be 10%. of Reaumur, equal to 540. of Farenheit. The temperature of the cave above mentioned fo nearly correfponds with this, that the difference may be ascribed to à difference of inftruments.

At the Panther gap, in the ridge which divides the waters of the Cow and Calf pasture, is what is called the Blowing cave. It is in the fide of a hill, is of about 100 feet diameter, and emits conftantly a current of air, of fuch force, as to keep the weeds proftrate to the distance of twenty yards before it. This current is strongest in dry, frofty weather, and in long fpells of rain weakeft. regular infpirations and expirations of air, by caverns and fiffures, have been probably enough accounted for, by fuppofing them combined

with intermitting fountains; as they muft of courfe inhale air while their refervoirs are emptying themfelves, and again emit it while they are filling. But a conftant iffue of air, only varying in its force as the weather is dryer or damper, will require a new by pothefis. There is another blowing cave in the Cum berland mountain, about a mile from where it croffes the Carolina line. All we know of this is, that it is not conftant, and that a fountain of water iffues from it.

The Natural Bridge, the moft fublime of nature's works, though not comprehended under the prefent head, muft not be pretermitted. It is on the ascent of a hill, which feems to have been cloven through its length by fome great convulfion. The fiffure, juft at the bridge, is, by fome admeafurements, 270 feet deep, by others only 205. It is about 45 feet wide at the bottom, and go feet at the top; this of course determines the length of the bridge, and its height fromthe water. Its breadth in the middle, is about 60 feet, but more at the ends, and the thicknefs of the mass, at the fummit of the arch, about 40 feet. A part of this thickness is conftituted by a coat of earth, which gives growth to many large trees.: 3 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 chord of the arch, is many times longer than the transverse. Though the fides of this bridge are provided in fome parts with a parapet of fixed rocks, yet few men have

the

the refolution 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 from below is delightful in an equal extreme. It is impoffible for the emotions arifing from the fublime, to be felt beyond what they are here: fo beautiful an arch, fo elevated, fo light, and fpringing as it were up to heaven! the rapture of the fpectator is really indefcribable! The fiffure continuing narrow, deep, and ftraight, for a confiderable distance above and below the bridge, opens a fhort but very pleafing view of the north mountain on one fide, and Blue ridge on the other, at the distance each of them of about five miles. This bridge is in the county of Rockbridge, to which it has given name, and affords a public and commodious paffage over a valley, which cannot be croffed elsewhere for a confiderable diftance. The ftream paffing under it is called Cedar creek. It is a water of James River, and fufficient in the dryeft feafons to turn a grift-mill, though its fountain is not more than two miles above.* QUERY

Don Ulloa mentions a break, fimilar to this, in the province of An garez, in South America. It is from 16 to 22 feet wide, 11 feet deep, and of 1.3 miles continuance, Englifh measure. Its breadth at top is not fenfibly greater than at bottom. But the following fact is remarkable, and will furnish fome light for conjecturing the probable origin of our nat rad bridge. Efta caxa, ó cauce está cortada en péna viva con tanta pre

cifion,