Page:Notes on the State of Virginia (1802).djvu/41

Rh Another of theſe caves is near the North mountain, in the county of Frederick, on the lands of Mr. Zane. The entrance into this is on the top of an extenſive ridge. You deſcend 30 or 40 feet, as into a well, from whence the cave then extends, nearly horizontally, 400 feet into the earth, preſerving a breadth of from 20 to 50 feet, and a height of from 5 to 12 feet. After entering this cave a few feet, the mercury, which in the open air was at 50° roſe to 57° of Farenheit's thermometre, anſwering to 11° of Reaumur's, and it continued at that to the remoteſt parts of the cave. The uniform temperature of the cellars of the obſervatory of Paris, which are 90 feet deep, and of all ſubterraneous cavities of any depth, where no chymical agents may be ſuppoſed to produce a factitious heat, has been found to be 10° of Reaumur, equal to 54° 30′ of Farenheit. The temperature of the cave above mentioned ſo nearly correſponds with this, that the difference may be aſcribed to a difference of inſtruments.

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 ſide of a hill, is of about 100 feet diameter, and emits conſtantly a current of air, of ſuch force, as to keep the weeds proſtrate to the diſtance of twenty yards before it. This current is ſtrongeſt in dry, froſty weather, and in long ſpells of rain weakeſt. Regular inſpirations and expirations of air, by caverns and fiſſures, have been probably enough accounted for, by ſuppoſing them combined with intermitting fountains; as they muſt of courſe inhale air while their reſervoirs are emptying themſelves; and again emit it while they are filling.