as glass. Beautifully brilliant were the prismatic colours reflected from the varied surface of the ice when the torches flashed suddenly upon them as they passed from cave to cave. Around, above, beneath, everything was of solid ice, and being unable to stand on account of its slippery nature, they slid, or rather glided mysteriously, along the glassy surface of this hall of spells. In one of the largest compartments the icicles had reached the floor, and gave the idea of pillars supporting the roof.' *

Rocks of volcanic formation seem to afford favourable opportunities for the congelation of water. Ice-caves are found in Mount Etna, on the Peak of Teneriffe, and among the lava-currents of Iceland.+ Scrope visited one of these natural glacières near the village of Roth, in the neighbourhood of Andernach, on the Rhine. It formed the mouth of a deep fissure in a current of basalt derived from an ancient volcanic cone above it, and its floor was covered with a crust of ice at the time of his visit, about noon on a very hot day in August.

The phenomenon of wind-grottoes is analogous to that of ice-caves, and not seldom associated with it. Here cold currents of air, increasing in violence as the day is hotter, are found to blow from the interstices of rocks. One of the most celebrated of these Eolian caverns is found near Terni in Italy. The entrance is closed by an old gate, through the crevices of which the wind issues with a rustling noise, while in the grotto itself the current is sufficiently strong to extinguish a torch. The proprietors of some neighbouring villas have put the phenomenon to an ingenious use. Leaden pipes, branching out from the grotto, convey on sultry summer days an agreeable coolness through masks of gypsum with wide distended mouths, which are fixed in the walls of the apartments.

The small town of Roquefort in France has been renowned ever since the time of the Romans for the delicious flavour of its cheese, which is said to owe its excellence to the cool cellars in which it is matured. These are excavated on the northern slope of a great chalk plateau, and communicate with numerous fissures in the rock, from which air-currents * Burslem, A Peep into Toorkistan.' The Cave of Surtshellir.

stream forth of so low a temperature as to cause a thermometer marking +23° R. in the shade, and in the external atmosphere, to fall to +4° R. when exposed to their influence. The cellars are so valuable that one, which cost 12,000 francs in construction, sold for 215,000 francs.

In times of ignorance, superstition could not fail to attach its fables to the phenomenon of wind-grottoes. A cave near Eisenach was supposed to be the seat of purgatory, and popular credulity or terror willingly transformed the sounds produced by the rushing air-currents into the wailings of tormented souls.

Fortunately, modern science affords us a more satisfactory explanation of the phenomenon. Pictet represents the case of a cave with cold currents of air to be much the same as that of a mine with a vertical shaft ending in a horizontal gallery, of which one extremity is in communication with the open air, at a point much lower, of course, than the upper extremity of the shaft. The cave or wind-hole corresponds to the horizontal gallery, and the various fissures in the rock take the place of the vertical shaft, and communicate freely with the external air. In summer the columns of air contained in these fissures assume nearly the temperature of the rock in which they rest-that is to say, the mean temperature of the district; and therefore they are heavier than the corresponding external columns of air which terminate at the mouth of the cave. The consequence is, that the heavy cool air descends from the fissures, and streams out into the cave, appearing as a cold current, and the hotter the day-that is, the lighter the columns of external air-the more violent will be the disturbance of equilibrium, and therefore the more palpable the current.

The evaporation which takes place as the air-currents descend through the moist rock-fissures likewise tends to lower their temperature. Several naturalists have attempted to explain the phenomena of ice-caves in a similar manner, as being produced by cold currents still further refrigerated by the evaporation caused in the moist and porous rocks through which they pass. But to this theory there are weighty objections, as in many ice-caves there is no current whatever, and

in all the cases of cold air streams investigated or mentioned by De Saussure, the lowest temperatures observed were still considerably above the freezing-point, and consequently incapable of converting water into ice.

Mr. Browne believes that, in many cases, the phenomenon may be satisfactorily accounted for by the position and surroundings of the caves in which it occurs; though, no doubt, cold currents and evaporation may often have an influence in maintaining the low temperature of ice-caves.

In every one of the fourteen natural glacières which he visited, the level at which the ice was found was considerably below the level at the entrance of the cave; so that, on ordinary principles of gravitation, the heavy cold air within could not be dislodged by the lighter warm summer air without. Heat naturally spreads very slowly in a cave like this; and even when some amount of heat does reach the ice, the latter melts but slowly, for ice absorbs 60° C. in melting; and thus when ice is once formed, it becomes a material guarantee for the permanence of cold in the cave.

Another means for preventing the encroachments of the hotter seasons is the dense covering of trees and shrubs, which, in the case of many of the glacières, shields their entrance or their roof from the rays of the sun, and thus keeps off the effects of direct radiation. Mr. Browne found all the glacières that came under his observation thus protected, with the single exception of that of St. Georges, where, in consequence of an incautious felling of wood immediately near the mouth, trunks of trees had been laid horizontally over it, to prevent the rays of the sun from striking down on to the ice. He moreover invariably found that the entrances to the caves were more or less sheltered against all winds-a very important condition, as air-currents from without would infallibly bring in heated air, in spite of the specific weight of the cold air stored within. There can be no doubt, too, that the large surfaces which are available for evaporation have much to do with maintaining a somewhat lower temperature than the mean temperature of the place where the cave occurs. Another great advantage which some glacières possess must be borne in mind, namely, the collection of snow at the bottom of the pit in which the

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entrance lies. This snow absorbs, in the course of melting, all the heat which strikes down by radiation, or is driven down by accidental turns of the wind; and the snow water thus forced into the cave will, at any rate, not seriously injure the ice.

It is easy to understand how, in caves thus protected against the influence of summer heat, a great part of the ice accumulated during the winter may be preserved, and

that, for an explanation of the phenomenon, it is by no means necessary to have recourse to cold blasts descending from the interior of the rock in which they are situated. It is indeed a common belief that the ice-caves are colder in summer than in winter, and consequently contain a greater abundance of ice during the former season; but this belief may well be considered as one of those popular fallacies, which-though, by dint of repetition, they come to be common articles of faith-have in fact no substantial proofs to support them.

CHAPTER XVIII.

ROCK TOMBS AND CATACOMBS.

Biban-el-Moluk, the Royal Tombs of Thebes-The Roman Catacombs-Their Extent---Their Mode of Excavation-Touching Sepulchral Inscriptions-Antony Bosio, the Columbus of the Catacombs-The Cavaliere di Rossi-The Catacombs of Naples and Syracuse-The Catacombs of Paris.

HE remoteness of caves and grottoes from the busy haunts of life, their eternal silence and their nightly gloom, have ever pointed them out as fit resting-places for the dead. From the earliest times they have been used as sepulchral vaults, and where nature neglected to hollow out the rock, it has often been excavated for this purpose by the hand of

man.

Thus the Pharaohs of Egypt rested not in temples and mausoleums reared in the heart of cities, but they chose the desert-ravine for their sepulchre, and hid their tombs in deep excavations in the earth.

A more impressive scene can hardly be imagined than that which is afforded by these splendid memorials. Of all such monuments which still mark the site of ancient Thebes, perhaps none are more striking to the traveller than the royal tombs-Biban-el-Moluk-which the pride of monarchs, whose very name is now a mystery, excavated four thousand years ago in the bosom of the Libyan mountains.

The next morning at daybreak,' says Warburton,* 'we started for the Tombs of the Kings. I was mounted on a fine horse, belonging to the sheikh of the village, and the cool air of the morning, the rich prospect before us, and the cloudless sky, all conspired to impart life and pleasure to my relaxed and languid frame. I had been for a month almost confined to my pallet by illness, and now, mounted on a gallant

*The Crescent and the Cross.'