Few have resolution enough to walk to the parapet, in order to peep over it. But if the view from the top is painful and intolerable, that from below is pleasing in an equal degree. The beauty, elevation, and lightness of the arch, springing as it were up to heaven, present a striking instance of the graceful in combination with the sublime. This great arch of rock gives the name of Rock-bridge to the county in which it is situated, and affords a public and commodious passage over a valley which cannot be crossed elsewhere for a considerable distance. Under the arch, thirty feet from the water, the lower part of the letters G. W. may be seen, carved in the rock. They are the initials of Washington, who, when a youth, climbed up hither, and left this record of his adventure. We have several examples of the disappearance of rivers, and their emergence after pursuing for some distance a subterranean course. In these cases a barrier of solid rock, overlaying a softer stratum has occurred in their path; and the latter has been gradually worn away by the waters, and a passage been constructed through it. Thus the Tigris, about twenty miles from its source, meets with a mountainous ridge at Diglou, and, running under it, flows out at the opposite side. The Rhone, also, soon after coming within the French frontier, passes under ground for about a quarter of a mile. Milton, in one of his juvenile poems, speaks of the “Sullen Mole, that runneth underneath;” and Pope calls it, after him, the “Sullen Mole, that hides his diving flood.” [Illustration: =Natural Bridge, Virginia.=] The Hamps and the Manifold, likewise--two small streams in Derbyshire--flow in separate subterraneous channels for several miles, and emerge within fifteen yards of each other in the grounds of Ilam Hall. That these are really the streams which are swallowed up at points several miles distant has been frequently proved, by watching the exit of various light bodies that have been absorbed at the swallows. At their emergence, the waters of the two rivers differ in temperature about two degrees--an obvious proof that they do not anywhere intermingle. On the side of the hill, which is overshadowed with spreading trees, just above the spot where the streams break forth into daylight, there is a rude grotto, scooped out of the rock, in which Congreve is said to have written his comedy of the “Old Bachelor,” and a part of his “Mourning Bride.” In Spain, a similar phenomenon is exhibited by the Guadiana; but it occurs under different circumstances. It disappears for about seven leagues--an effect of the absorbing power of the soil--the intervening space consisting of sandy and marshy grounds, across which the road to Andalusia passes by a long bridge or causeway. The river reappears with greater power, after its dispersion, at the Ojos de Guadiana--the Eyes of the stream. [_To be continued._