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Page:Popular Science Monthly Volume 54.djvu/273

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exploration in a dry season might reveal many very interesting chambers.

In the cave of Rabanel, the first well, which ends in a talus of fallen stones, furnishes an instance of a vertical fissure grafted, if we may use the word, upon an interior grotto that already existed. A stream runs through this grotto which falls into a second well twenty-six metres, and is then lost in smaller passages so nearly stopped up with earth that we were not able to follow it through its course of about a mile till it comes out at the Brissac spring.

The cave of Trebiciano, in Istria, near Trieste, the deepest known, has a total depth of more than a thousand feet. It is not, however, entirely natural, but is composed of numerous vertical fissures which lead, at about eight hundred and fifty feet below the surface, to a large cavern, at the bottom of which flows the subterranean river Recca. The fissures do not naturally communicate directly with one another, but the engineer Lindner was commissioned in 1840-'41 by the city of Trieste to construct for the municipality a supply of potable water from the underground streams, and after eleven months of labor made artificial connections between the different parts of the chasm.

These vertical pits are formed by the wearing down, from the top, by the waters which become ingulfed in them. This mode of their formation was demonstrated to me in 1895, when I was in Great Britain under a commission from the French Minister of Instruction. I then explored several caves in which the rivers were still running, and satisfied myself that the pits were simply absorbing wells. Such wells are not effective now in southern France and Austria, but in northern Europe, where rain is more abundant, they are still operative. I found the plainest evidence of this fact in Yorkshire, at the Gaping Ghyll, Ingleborough, where a river precipitates itself at one leap one hundred metres under the earth. English investigators and travelers had tried without success to descend into it in 1845, 1870, and 1894, having conquered only about one hundred and ninety-five feet of its total depth of two hundred and twenty-nine feet. It took me twenty-five minutes to go down upon a rope ladder which was suspended in the midst of the cascade. Fortunately, the pit had the daylight to the very bottom—a wonderful spectacle, compensating me for all my trouble and the long douche bath which greeted me at the end of the descent, where stretched an immense Roman nave nearly five hundred feet long, eighty feet wide, and ninety feet high, without any sustaining pillar. From the middle of the roof of this colossal cavern fell the cascade in a great nimbus of vapor and light—a wonderful fantastic scene, such as Gustave Doré or Jules Verne could never have imagined.