Imágenes de páginas
PDF
EPUB

rare and interesting objects. Among these we may reckon the Sulphur-mountains; one of which, on the south side of the Guldbringe district, is described by Sir George Mackenzie At the foot of the mountain there was a bank composed of clay and sulphur, with steam issuing from all parts of it. From a ridge immediately above it; under which was a deep hollow, a profusion of vapour arose, and a confused noise was heard, of boil- ing and splashing, joined to the roaring of steam 'escaping from crevices in the rock. The opposite side of the mountain was covered with sulphur, and clay of a white or yellowish colour. From whatever spot the sulphur was removed, steam instantly escaped ; and, in many places, the sulphur was so hot that they could scarcely touch it. From the smell it appeared that the steam was mixed with a small quantity of sulphurated hydrogene gas.

When the thermometer was sunk a few inches into the clay, it rose almost to the boiling point. At the bottom of the hollow, they found a caldron of mud and water boiling with great vehemence. The mud was in constant agitation, and often thrown up to the height of 6 or 8 feet." In some places the quantity of sulphur was very great, and formed a smooth crust, beautifully chrystallized, and from a quarter of an inch to several inches in thickness: The violence with which the steam issues through the crevices of the rock is in some places so great, that the noise may be heard at the distance of several miles. The visit to this place was not without danger. The sensations, says Sir George, of a person standing on a support which fer bly sustains him over an abyss where fire and brimstone are in dreadful and incessant action ;-having before him tremena dous proofs of what is going on beneath ; -enveloped in thick vapours, and stunned with thundering noises ;-are hardly to be conceived by one who has not experienced them.

Their next visit was to the hot springs of Geyser, situated considerably inland, and more than 60 miles east of Reikiavic. They are in a valley of considerable size, and on the side of the river. The principal fountain, the great Geyser, is in the middle of a sinali eminence, which extends all round it, and is about 7 feet high. The basin in the middle is of an oval form, 56 feet in the longest diameter, and 46 in the shortest. This basin, when they arrived, was full of hot water, with a little running out at one side. After examining some of the fountains in the neighbourhood, of which there are several, they returned to the great Geyser, where they were alarmed by a sound like the distant discharge of artillery, and the shaking of the ground. The water, after heaving several times, suddenly arose in a large co

lumn,

the a:

Bumn, accompanied by clouds of steam, to the height of 10 or 12 feet. The column then seemed to burst, and, sinking down, caused the water to everflow in considerable quantity. This was followed by a succession of jets, to the number of 18, some of them rising to the height of 50 feet. After the last of these, the water disappeared from the basin, and súnk within a pipe about 10 feet wide, which is in the centre of it. The perpendicular depth of the basin is about 3 feet, and that of the pipe appeared to be about 60. At 29 minutes past 6 in the evening, the pipe was full, and the water being within reach, its temperature was found to be 209o. No great jet, however, took place for a long time. We pitched our tents,' says Sir George, at the distance of about 100 yards from the Geyser, and determined to keep watch by turns during the night. About 4 in the morning, Mr Bright, who happened to have the wateh, gave larm ; and we saw water thrown up, and steam issuing with a tremendous noise, from a place within 50 yards of us, which we had not before remarked. There was fittle water; but the force with which the steam escaped, produced a white column of spray

and vapour at least 60 feet high. We enjoyed this astonishing and beautiful spectacle till 7 o'clock, when it gradually clisappeared. We conjectured this to be the fountain which Sir. John Stanley has called the New Geyser."

The beautiful and variegated petrifactions which surround the Geysers have been often described; the leaves of birch and willow are seen converted into white stone, in a state of the most perfect preservation, every fibre being entire. Grass, rushes, and masses of peat, are in the same condition. On the outside of the mouth of the Geyser, the depositions, owing to the splashing of the water, are rough, and have been compared to the heads of cauliflower. The inside of the basin is comparatively smooth ; and the matter forming it is more compact and dense than the exterior crust. Sir George and his friends carried off a great quantity of these curious specimens ; of which he has presented a very fine collection to the Royal Society of Edinburgh.

It was not till the night after the explosion just mentioned, that they had the satisfaction to see the Great Geyser display its utmost magnificence. This happened about midnight. At that season of the year there was light enough to render. the. whole visible; and the effect was more striking, perhaps, from the partial obscurity. The fountain threw up a succession of magnificent jets, the highest of which reached to the height of 90 feet. The internal structure necessary to produce those singular al

ternatione

ternations of activity and rest, even supposing a sufficiency of water and of heat to be provided, is not easily conceived. That the elasticity of vapour is the great agent, and that the Geyser is a natural steam engine cannot be questioned. A great quantity of steam is always thrown up with the water; and, in some of the smaller fountains, bursts of steam are sometimes thrown up through the water, the ground at the same time being felt to tremble all round. Sir George has given a description of the manner in which he thinks that the Geyser may be produced. He conceives a cavity in the heart of a rock to be supplied withwater by percolation ; while a pipe, first bending to a lower level, sends up a perpendicular shaft, which opens at the surface. The lower part of this cavity, and part of the shaft or neck, being filled with water, if heat continue to be applied, a quantity of steam will fill the part of the cavern between the surface of the water and the roof. The steam, thus enclosed, if the heat be continued, will increase in temperature, and acquire elasticity sufficient to force up the water in the shaft, and to throw it to a great height in the air. This is certainly a mechanism by which appearances similar to the Geyser may be produced ; though, whether it be the actual process of nature, we may never be able to discover. Sir George observed a fountain, to which he gives the name of the Alternating Geyser. It consisted of two jets from different points; and the one began to rise always when the other ceased. He has proposed it as a problem in hydraulics, to contrive the means by which an alternation of this kind might be produced without the use of valves, which he thinks it probable that nature does not employ. We see no reason, however, for thinking that valves are not among the resources 'which nature has in store in the bowels of the earth. If we suppose a perpendicular pipe or shaft in which there is a contraction, and that above that contraction there lyes a large round and smooth stone, of size sufficient to shut the contracted part of the pipe, but not completely to fill the superior or wider part, we have a valve of a very perfect kind, and one which, in strict conformity to analogy, we may imagine nature to possess. " By means of such a valve, the jet of the Geyser might be produced, without the bent pipe in the description just given. It would be no objection to this theory, that such a valve as is here supposed, must be subject to continual waste, and must in time be insufficient for the purpose. The changes that take place in these fountains seem to show, that the mechanism by which they are produced is not the inost permament.

We have already mentioned the extensive promontory that bounds the Gulf of Faxé on the north. This promontory is very mountainous, and the summits of the mountains are mostly covered with snow. The name given to a mountain of this sort, in the Icelandic language, is Jokul; and the highest of these, situated at the very western extremity of the pronontory, is called Snæfell-Jokul. Mr Holland and Mr Bright ascended to the top of the Jokul ; and a very lively account of their excursion is given from the journal of the latter.

Having procured a guide, which was no easy matter, (such is the kind of superstitious terror that the Icelanders have for this mountain), they began to ascend; and, after walking two hours over a barren surface that at every step became more destitute of vegetation, they reached the snow. At first, the spow yielded to the impression of the foot; but by and by, it became harder, and the steepness so great, as to render ihe ascent difficult. The snow was frequently intersected by deep and wide chasms, the passage of which was difficult, and not a little dangerous. At last, they reached one of the three summits; but the highest point of all, about 100 feet higher, was rendered inaccessible by a deep chasm that intervened. At the highest point to which they reached, the thermometer stood at 34, and on the snow, at 32; it was then about 3 o'clock. On the sea-shore, at 11 in the morning, the thermometer was at 58. After having enjoyed a fine view of the coast, and the adjacent mountains, they descended, much pleased with their excursion, and none more than the guide, who found it difficult however to persuade his countrymen that he had really been on, the top of Snæfell Jokul ; such is the superstitious reverence or fear with which this mountain is regardled. We regret that the. travellers had no barometer, as the ascertaining of the lower limit of the snow, is a material point, not merely in the natural history of this country, but of climate in general, as fix

as fixing the limit of congelation at the entrance into the polar circle. The barometer is an instrument very liable to such accidents as had deprived our travellers of theirs ; and it would be well if the resources were perfectly understood by which the want of a barometer may, in some measure be supplied.

If a traveller be provided with a quadrant, or any instrument for measuring vertical angles, his best and easiest method is to take the angle of altitude from a point, of which the distance from the mountain can be measured on a map. Had our travellers, for instance, taken such an observation at Olafsvic, on the seashore, the place from which they set out, and had they repeated the same at a point on the opposite coast, when they were on

the We would very

the other side of the mountain, the mean between these two computations of the height could not have failed of coming very near the truth. The corrections for curvature and refraction might be applied as directed in the note. much recommend this method to travellers, who feel an interest in measuring the elevation of the ground over which they pass, and are not provided with barometers.

Whenever mountains, where snow is perpetual, are described, a question occurs which it is always material to resolve, viz. Whether the covering consists simply of snow, or if it is what. is properly called a glacier ? Travellers do not seem to be always aware of the difference between these two ways in which frost takes possession of the tops and declivities of mountains. In the one, the substance is real snow, perhaps much indurated, but still retaining its granular texture, and its white colour. In the other, the snow is first soaked with rain, which afterwards freezes, and converts the whole into ice. This last is the glacier; it is an emanation from the snow, and constitutes a river of ice, if we may so call it, descending from the great lake of congealed water which rests on all the summits that penetrate into the region of perpetual frost, Wherever we would define the lower boundary of that region, and trace through the atmosphere the line that separates animate from inanimate nature, this distinction is necessary to be kept in view. Mr Bright has not stated directly any opinion on this subjects but his account seems to exclude the idea of a glacier. Supposing this to be fact, the height at which they fell in with the snow, according to Kirwan's table, + was 2516 feet above the level of the sea.

At Olafsvic, the nearest village to the mountain, the sun was seen both to rise and set in the sea ; and, on the 5th of July, Mr Holland observed that he was 2h 35' under the horizon, the latitude being 64° 58'. Calculating from the instant of the upper. {imb of the sun descending below the horizon, to the instant when the same limb appears above it, the time, paying no re

gard

* Multiply the horizontal distance in feet by the tangent of the observed angle, it will give the height nearly, to be corrected for the curvature of the earth, and the refraction of light, thus :

Square the horizontal distance reckoned in English miles ; two." thirds of the amount is the correction in feet, to be added to the height already found on account of the curvature of the earth,

From the height thus corrected, subtract one-seventh of the last correction, and it will give the height corrected, both for curvature and refraction.

Estimate of the temperature of different Latitudes,

[ocr errors]
« AnteriorContinuar »