deep fissures and yawning gulfs, with rugged, broken edges, while in other places are jagged heaps of rock that seem not to have been sufficiently heated to flow, but were forced upwards by a mighty agency, and then rent and shattered to their centre. The rock is filled with little pits and protuberances like air-bubbles; it is as hard as flint, and emits a sharp metallic sound when struck. I did not observe any approach to columnar or crystallised basalt."

I am not aware that among the phenomena exhibited by the volcanic districts in the south of Europe, with which I am familiar, either around Naples and Vesuvius, or within the Isle of Ischia, or in the vicinity of the more glorious Etna, there are any of the like extent and magnitude, nor did I see there present any such deep chasms and fissures* as those observed in the Lejah and Safah; but if we turn to that entirely volcanic island in the north-west of Europe, and just within its geographical range-Iceland-we shall perceive the nearest parallel to some of those phenomena now under notice, especially to the enormous clefts or fissures, named in Icelandic gias. "Near Thingvalla," Sir George M'Kenzie has written, in his able "Travels in Iceland," "we entered a deep and frightful fissure called Almannagiau. This has been formed, with many others of smaller dimensions, and another large one which runs parallel to it at a considerable distance, by the sinking of the ground during some of those terrible convulsions which have shaken Iceland to its foundations. The whole rock bears marks of having been affected by fire. We came suddenly upon the brink of the precipice, and were turning aside from a scene so fearful, when we were told that we must descend."

Pliny Miles in 1852, just before arriving at Thingvalla, "while jogging along on the level ground, came suddenly upon the brink of an immense chasm, 150 feet deep, and about the same in breadth." This, he says, is "one of those seams or rents in the earth common in Iceland; originally a crack in a bed of lava. Its precipitous sides and immense

* But in passing through Masca Lucia, between Nicolosi and Catania, on my descent from Etna, I learned that a large and deep fissure existed near that place. I do not know whether it be in a lava-bed or not.

NEW SERIES.-VOL. XI. NO. II.-APRIL 1860.

depth seemed at once a bar to our progress; and without a bridge over it, or ropes or wings, we saw no way of getting along without going round it. Without seeing either end, and wondering how we were to get round it, we were told we must go through it. At length we made our way to the bottom. We were in a deep chasm or defile, the wall on the west side being over 100 feet high, and on a level with the country behind it. The wall on the east side was lower, and beyond this wall the country was on a level with the bottom where we were. This chasm is called the Almannagja, or Allmen's-Cave.'” Again, the same traveller adds, in another district, "We travelled several miles over a most desolate volcanic region, completely covered with lava rocks, scoriæ, and volcanic sand. Like all the lava-covered country, it was broken up in huge, irregular masses, and very cavernous—in some places showing caves 30 or 40 feet deep. No description or picture will give a good idea of the old lava on the surface of the ground, to a person who has never been in a volcanic country. Not the roughest limestone region I have ever seen will bear the slightest comparison with the lavacovered districts (near two-thirds of the surface) of Iceland. In written descriptions of volcanic regions we often see mention made of 'streams of lava.' These streams in other countries are usually down the sides of mountains, but here in Iceland they extend for miles along the surface of the level ground, and we are puzzled to know where they came from, for usually we see no crater or mountain anywhere near. I have seen these streams' standing up in bold relief, a black, rough, horrid mass from 10 to 100 feet deep, several hundred yards wide, and one or two miles in length."

Also in 1853, an amusing yacht-voyager writes, near the lake Thingvalla, "we found ourselves upon the brink of one of those extraordinary chasms which the natives call by the euphonious name of gja. And truly no name can be rugged or break-jawed enough to express the horror of one of these break-neck gjas. The craters in the moon are the only familiar objects to which the general reader may be referred, to catch a glimpse of their astounding reality. Or imagine two cinders, about a couple of miles long, placed a little way

apart, and viewed through a "microscope of extra power," and you will have some notion of this Hrafnagja, or "Raven Chasm," on the brink of which our little ponies now stood, and across which our unflinching pathway lay." And describing Thingvalla, he continues: "It is in immediate proximity to the most wonderful of all the gjas, called Almannagja. This is a huge chasm some miles in length, stretching in a northerly direction from the head of the Thingvalla lake. It divides the great plain into two distinct parts, of which the westernmost is of a much higher level than the eastern one. And it would seem as if this yawning chasm had been made by the sudden upraising of the western portion of the plain by means of some subterranean agency. It should be remarked, that the western face of the chasm is precipitous, while the eastern and lower side has a gradual slope, as if it had fallen away from the other during the upheaving." And leaving that lake, and passing over a large lava-field, he "came upon a curious crater-like mound, composed of crisp cinder, of all manner of bright colours. On reaching it we found it was a mere shell, hollow within, and we could look down from the top into a spacious cavern, extending far on all sides. Indeed, we could not but come to the conclusion that the whole of this lava-field is more or less hollow; for many were the great caverns we passed as we rode along."

And in 1855, Mr Robert Chambers, in his pleasing “Tracings of Iceland," describes the Almannagiau as "but one of the lateral rents resulting from subsidence. Along the rising ground to the east appear four more, one above the other, and all parallel." Again, he mentions the Hrafnagiau, as "a terrific abyss, which extends for two or three miles, with a width of from ten to thirty feet, and would be totally impassable, but for a few blocks which have fallen in at a certain place, and to some extent filled up the gulf. From the general narrowness, irregularity, and darkness of this chasm, it would form an admirable retreat for robbers. The surface continues for miles to exhibit the same fractured character, with less appalling effects. There are also short minor subsidences, leaving a piece of surface forty feet or so below the rest, while, at the ends, a thin superficial crust having re

mained at the original level, antres vast' have been formed."

I need not, however, insert any further descriptions of awful fissures, chasms, rents, caverns, or basaltic and lava masses, which occur in other portions of Iceland, as parallels to those in the Lejah and Safah, that have been so well detailed by Mr Porter and Mr Graham.

Proceeding westwards from the Safah," the group of conical hills called E' Tellul," as Mr Porter describes it, appears on the north, and "is about fifteen miles in length," running, like the Safah, near north and south. The Tell named Dukweh is the highest cone-shaped peak, and rises nearly in the centre of the Tellul. From its southern end to Gebel Hauran there extends a vast plain.

Lastly, on the plain beyond, to the west, is seen the southernmost of the three black ruins called E' Diûra-that is to say, "The Convents." It is a square tower, built of hard basalt, and is most likely a border fortress; and, in fact, Ammianus Marcellinus (lib. xiv.) mentions that the early inhabitants built castles and strong forts as defences against the incursions of the neighbouring hostile tribes.

And, in concluding this imperfect communication, I must express an earnest hope that some enterprising British geologist would undertake a scientific journey throughout the mountains of Haurân, and the volcanic districts in the neighbouring Harrah, the Tellul, and the Safah, in order to determine with more exactness the remarkable natural phenomena and different formations which there present themselves.

On the Application of certain Laws of Heat and Combustion to the Use and Economy of Fuel. Being the substance of a Paper read before the Aberdeen Philosophical Society, 11th February 1859. By ALEXANDER D. MILNE. Part II.

Of the Hydrogen Element in ordinary Fuel, and the influence it exercises on Combustion.

Common coal generally contains about 80 per cent. of car

bon, and only 3 to 5 per cent. of hydrogen. While, therefore, we are justified in applying generally the laws which regulate the combustion of carbon to the use of common coal, we cannot overlook the influence of hydrogen, as it modifies greatly the action of the other element. This will be readily granted, because, 1st, Its heating power, weight for weight, is three to four times that of carbon; 2d, To its presence we owe the production of illuminating gas when coal is distilled; 3d, It is also solely and entirely the origin of flame, and smoke or soot, in the combustion of coal. most important considerations.

These are all

In Table II. we give the particulars of its combustion. We would draw special attention to column 9, in which we deduct 8685° Fahr. from its heating power when used for ordinary purposes, such as raising of steam, and, indeed, for all purposes where the terminal temperature exceeds 212°, because above 212 that amount of caloric (8685°) is latent, and for all heating and pyrometrical ends may be regarded as nonexistent. If we cool the products below 212 this caloric becomes sensible; and this, Andrews and other experimenters did. In several treatises on fuel this allowance is not made; the heating power is assumed, for practical purposes, as 60,840, and the temperature upwards of 4000° Fahrenheit, while in reality they are only 52,155° and 3476° respectively.

We might have given for hydrogen a table precisely similar to Table III., but it is unnecessary. The same laws apply to both elements, with this difference: In carbon, the original products of combustion (carbonic acid and nitrogen) have 2596 specific heat; and all successive additions of air in excess have 2669 specific heat-almost the same in both cases. But in burning hydrogen, the original products (vapour of water and nitrogen) have 4191 specific heat, which is much higher than that of air. This necessitates a change in the formulæ for hydrogen, to suit diminishing specific heat with every addition of draught. We shall simply give formula, assuming 52,155° as the heating power, and 3476° (in round numbers 3500°) as the initial temperature with 1 equivalent of draught.