by admin » Mon Jun 04, 2018 5:55 am
Part 2 of 2
In reflecting on the causes of the Atlantic currents, we find that they are much more numerous than is generally believed; for the waters of the sea may be put in motion by an external impulse, by difference of heat and saltness, by the periodical melting of the polar ice, or by the inequality of evaporation, in different latitudes. Sometimes several of these causes concur to one and the same effect, and sometimes they produce several contrary effects. Winds that are light, but which, like the trade-winds, are continually acting on the whole of a zone, cause a real movement of transition, which we do not observe in the heaviest tempests, because these last are circumscribed within a small space. When, in a great mass of water, the particles at the surface acquire a different specific gravity, a superficial current is formed, which takes its direction towards the point where the water is coldest, or where it is most saturated with muriate of soda, sulphate of lime, and muriate or sulphate of magnesia. In the seas of the tropics we find, that at great depths the thermometer marks 7 or 8 centesimal degrees. Such is the result of the numerous experiments of commodore Ellis and of M. Peron. The temperature of the air in those latitudes being never below 19 or 20 degrees, it is not at the surface that the waters can have acquired a degree of cold so near the point of congelation, and of the maximum of the density of water. The existence of this cold stratum in the low latitudes is an evident proof of the existence of an under-current, which runs from the poles towards the equator: it also proves that the saline substances which alter the specific gravity of the water, are distributed in the ocean, so as not to annihilate the effect produced by the differences of temperature.
Considering the velocity of the molecules, which, on account of the rotatory motion of the globe, vary with the parallels, we may be tempted to admit that every current, in the direction from south to north, tends at the same time eastward, while the waters which run from the pole towards the equator, have a tendency to deviate westward. We may also be led to think that these tendencies diminish to a certain point the speed of the tropical current, in the same manner as they change the direction of the polar current, which in July and August, is regularly perceived during the melting of the ice, on the parallel of the bank of Newfoundland, and farther north. Very old nautical observations, which I have had occasion to confirm by comparing the longitude given by the chronometer with that which the pilots obtained by their reckoning, are, however, contrary to these theoretical ideas. In both hemispheres, the polar currents, when they are perceived, decline a little to the east; and it would seem that the cause of this phenomenon should be sought in the constancy of the westerly winds which prevail in the high latitudes. Besides, the particles of water do not move with the same rapidity as the particles of air; and the currents of the ocean, which we consider as most rapid, have only a swiftness of eight or nine feet a second; it is consequently very probable, that the water, in passing through different parallels, gradually acquires a velocity correspondent to those parallels, and that the rotation of the earth does not change the direction of the currents.
The variable pressure on the surface of the sea, caused by the changes in the weight of the air, is another cause of motion which deserves particular attention. It is well known, that the barometric variations do not in general take place at the same moment in two distant points, which are on the same level. If in one of these points the barometer stands a few lines lower than in the other, the water will rise where it finds the least pressure of air, and this local intumescence will continue, till, from the effect of the wind, the equilibrium of the air is restored. M. Vaucher thinks that the tides in the lake of Geneva, known by the name of the seiches, arise from the same cause. We know not whether it be the same, when the movement of progression, which must not be confounded with the oscillation of the waves, is the effect of an external impulse. M. de Fleurieu, in his narrative of the voyage of the Isis, cites several facts, which render it probable that the sea is not so still at the bottom as naturalists generally suppose. Without entering here into a discussion of this question, we shall only observe that, if the external impulse is constant in its action, like that of the trade-winds, the friction of the particles of water on each other must necessarily propagate the motion of the surface of the ocean even to the lower strata; and in fact this propagation in the Gulf-stream has long been admitted by navigators, who think they discover the effects in the great depth of the sea wherever it is traversed by the current of Florida, even amidst the sand-banks which surround the northern coasts of the United States. This immense river of hot waters, after a course of fifty days, from the 24th to the 45th degree of latitude, or 450 leagues, does not lose, amidst the rigours of winter in the temperate zone, more than 3 or 4 degrees of the temperature it had under the tropics. The greatness of the mass, and the small conductibility of water for heat, prevent a more speedy refrigeration. If, therefore, the Gulf-stream has dug a channel at the bottom of the Atlantic ocean, and if its waters are in motion to considerable depths, they must also in their inferior strata keep up a lower temperature than that observed in the same parallel, in a part of the sea which has neither currents nor deep shoals. These questions can be cleared up only by direct experiments, made by thermometrical soundings.
Sir Erasmus Gower remarks, that, in the passage from England to the Canary islands, the current, which carries vessels towards the south-east, begins at the 39th degree of latitude. During our voyage from Corunna to the coast of South America, the effect of this motion of the waters was perceived farther north. From the 37th to the 30th degree, the deviation was very unequal; the daily average effect was 12 miles, that is, our sloop drove towards the east 75 miles in six days. In crossing the parallel of the straits of Gibraltar, at a distance of 140 leagues, we had occasion to observe, that in those latitudes the maximum of the rapidity does not correspond with the mouth of the straits, but with a more northerly point, which lies on the prolongation of a line passing through the strait and Cape St. Vincent. This line is parallel to the direction which the waters follow from the Azores to Cape Cantin. We should moreover observe (and this fact is not uninteresting to those who examine the nature of fluids), that in this part of the retrograde current, on a breadth of 120 or 140 leagues, the whole mass of water has not the same rapidity, nor does it follow precisely the same direction. When the sea is perfectly calm, there appears at the surface narrow stripes, like small rivulets, in which the waters run with a murmur very sensible to the ear of an experienced pilot. On the 13th of June, in 34 degrees 36 minutes north latitude, we found ourselves in the midst of a great number of these beds of currents. We took their direction with the compass, and some ran north-east, others east-north-east, though the general movement of the ocean, indicated by comparing the reckoning with the chronometrical longitude, continued to be south-east. It is very common to see a mass of motionless waters crossed by threads of water, which run in different directions, and we may daily observe this phenomenon on the surface of lakes; but it is much less frequent to find partial movements, impressed by local causes on small portions of waters in the midst of an oceanic river, which occupies an immense space, and which moves, though slowly, in a constant direction. In the conflict of currents, as in the oscillation of the waves, our imagination is struck by those movements which seem to penetrate each other, and by which the ocean is continually agitated.
We passed Cape St. Vincent, which is of basaltic formation, at the distance of more than eighty leagues. It is not distinctly seen at a greater distance than 15 leagues, but the granitic mountain called the Foya de Monchique, situated near the Cape, is perceptible, as pilots allege, at the distance of 26 leagues. If this assertion be exact, the Foya is 700 toises (1363 metres), and consequently 116 toises (225 metres) higher than Vesuvius.
From Corunna to the 36th degree of latitude we had scarcely seen any organic being, excepting sea-swallows and a few dolphins. We looked in vain for sea-weeds (fuci) and mollusca, when on the 11th of June we were struck with a curious sight which afterwards was frequently renewed in the southern ocean. We entered on a zone where the whole sea was covered with a prodigious quantity of medusas. The vessel was almost becalmed, but the mollusca were borne towards the south-east, with a rapidity four times greater than the current. Their passage lasted near three quarters of an hour. We then perceived but a few scattered individuals, following the crowd at a distance as if tired with their journey. Do these animals come from the bottom of the sea, which is perhaps in these latitudes some thousand fathoms deep? or do they make distant voyages in shoals? We know that the mollusca haunt banks; and if the eight rocks, near the surface, which captain Vobonne mentions having seen in 1732, to the north of Porto Santo, really exist, we may suppose that this innumerable quantity of medusas had been thence detached; for we were but 28 leagues from the reef. We found, beside the Medusa aurita of Baster, and the Medusa pelagica of Bosc with eight tentacula (Pelagia denticulata, Peron), a third species which resembles the Medusa hysocella, and which Vandelli found at the mouth of the Tagus. It is known by its brownish-yellow colour, and by its tentacula, which are longer than the body. Several of these sea-nettles were four inches in diameter: their reflection was almost metallic: their changeable colours of violet and purple formed an agreeable contrast with the azure tint of the ocean.
In the midst of these medusas M. Bonpland observed bundles of Dagysa notata, a mollusc of a singular construction, which Sir Joseph Banks first discovered. These are small gelatinous bags, transparent, cylindrical, sometimes polygonal, thirteen lines long and two or three in diameter. These bags are open at both ends. In one of these openings, we observed a hyaline bladder, marked with a yellow spot. The cylinders lie longitudinally, one against another, like the cells of a bee-hive, and form chaplets from six to eight inches in length. I tried the galvanic electricity on these mollusca, but it produced no contraction. It appears that the genus dagysa, formed at the time of Cook's first voyage, belongs to the salpas (biphores of Bruguiere), to which M. Cuvier joins the Thalia of Brown, and the Tethys vagina of Tilesius. The salpas journey also by groups, joining in chaplets, as we have observed of the dagysa.
On the morning of the 13th of June, in 34 degrees 33 minutes latitude, we saw large masses of this last mollusc in its passage, the sea being perfectly calm. We observed during the night, that, of three species of medusas which we collected, none yielded any light but at the moment of a very slight shock. This property does not belong exclusively to the Medusa noctiluca, which Forskael has described in his Fauna Aegyptiaca, and which Gmelin has applied to the Medusa pelagica of Loefling, notwithstanding its red tentacula, and the brownish tuberosities of its body. If we place a very irritable medusa on a pewter plate, and strike against the plate with any sort of metal, the slight vibrations of the plate are sufficient to make this animal emit light. Sometimes, in galvanising the medusa, the phosphorescence appears at the moment that the chain closes, though the exciters are not in immediate contact with the organs of the animal. The fingers with which we touch it remain luminous for two or three minutes, as is observed in breaking the shell of the pholades. If we rub wood with the body of a medusa, and the part rubbed ceases shining, the phosphorescence returns if we pass a dry hand over the wood. When the light is extinguished a second time, it can no longer be reproduced, though the place rubbed be still humid and viscous. In what manner ought we to consider the effect of the friction, or that of the shock? This is a question of difficult solution. Is it a slight augmentation of temperature which favours the phosphorescence? or does the light return, because the surface is renewed, by putting the animal parts proper to disengage the phosphoric hydrogen in contact with the oxygen of the atmospheric air? I have proved by experiments published in 1797, that the shining of wood is extinguished in hydrogen gas, and in pure azotic gas, and that its light reappears whenever we mix with it the smallest bubble of oxygen gas. These facts, to which several others may be added, tend to explain the causes of the phosphorescence of the sea, and of that peculiar influence which the shock of the waves exercises on the production of light.
When we were between the island of Madeira and the coast of Africa, we had slight breezes and dead calms, very favourable for the magnetic observations, which occupied me during this passage. We were never weary of admiring the beauty of the nights; nothing can be compared to the transparency and serenity of an African sky. We were struck with the innumerable quantity of falling stars, which appeared at every instant. The farther progress we made towards the south, the more frequent was this phenomenon, especially near the Canaries. I have observed during my travels, that these igneous meteors are in general more common and luminous in some regions of the globe than in others; but I have never beheld them so multiplied as in the vicinity of the volcanoes of the province of Quito, and in that part of the Pacific ocean which bathes the volcanic coasts of Guatimala. The influence which place, climate, and season appear to exercise on the falling stars, distinguishes this class of meteors from those to which we trace stones that drop from the sky (aerolites), and which probably exist beyond the boundaries of our atmosphere. According to the observations of Messrs. Benzenberg and Brandes, many of the falling stars seen in Europe have been only thirty thousand toises high. One was even measured which did not exceed fourteen thousand toises, or five nautical leagues. These measures, which can give no result but by approximation, deserve well to be repeated. In warm climates, especially within the tropics, falling stars leave a tail behind them, which remains luminous 12 or 15 seconds: at other times they seem to burst into sparks, and they are generally lower than those in the north of Europe. We perceive them only in a serene and azure sky; they have perhaps never been below a cloud. Falling stars often follow the same direction for several hours, which direction is that of the wind. In the bay of Naples, M. Gay-Lussac and myself observed luminous phenomena very analogous to those which fixed my attention during a long abode at Mexico and Quito. These meteors are perhaps modified by the nature of the soil and the air, like certain effects of the looming or mirage, and of the terrestrial refraction peculiar to the coasts of Calabria and Sicily.
When we were forty leagues east of the island of Madeira, a swallow* (* Hirundo rustica, Linn.) perched on the topsail-yard. It was so fatigued, that it suffered itself to be easily taken. It was remarkable that a bird, in that season, and in calm weather, should fly so far. In the expedition of d'Entrecasteaux, a common swallow was seen 60 leagues distant from Cape Blanco; but this was towards the end of October, and M. Labillardiere thought it had newly arrived from Europe. We crossed these latitudes in June, at a period when the seas had not for a long time been agitated by tempests. I mention this last circumstance, because small birds and even butterflies, are sometimes forced out to sea by the impetuosity of the winds, as we observed in the Pacific ocean, when we were on the western coast of Mexico.
The Pizarro had orders to touch at the isle of Lancerota, one of the seven great Canary Islands; and at five in the afternoon of the 16th of June, that island appeared so distinctly in view that I was able to take the angle of altitude of a conic mountain, which towered majestically over the other summits, and which we thought was the great volcano which had occasioned such devastation on the night of the 1st of September, 1730.
The current drew us toward the coast more rapidly than we wished. As we advanced, we discovered at first the island of Forteventura, famous for its numerous camels;* (* These camels, which serve for labour, and sometimes for food, did not exist till the Bethencourts made the conquest of the Canaries. In the sixteenth century, asses were so abundant in the island of Forteventura, that they became wild and were hunted. Several thousands were killed to save the harvest. The horses of Forteventura are of singular beauty, and of the Barbary race.—"Noticias de la Historia General de las Islas Canarias" por Don Jose de Viera, tome 2 page 436.) and a short time after we saw the small island of Lobos in the channel which separates Forteventura from Lancerota. We spent part of the night on deck. The moon illumined the volcanic summits of Lancerota, the flanks of which, covered with ashes, reflected a silver light. Antares threw out its resplendent rays near the lunar disk, which was but a few degrees above the horizon. The night was beautifully serene and cool. Though we were but a little distance from the African coast, and on the limit of the torrid zone, the centigrade thermometer rose no higher than 18 degrees. The phosphorescence of the ocean seemed to augment the mass of light diffused through the air. After midnight, great black clouds rising behind the volcano shrouded at intervals the moon and the beautiful constellation of the Scorpion. We beheld lights carried to and fro on shore, which were probably those of fishermen preparing for their labours. We had been occasionally employed, during our passage, in reading the old voyages of the Spaniards, and these moving lights recalled to our fancy those which Pedro Gutierrez, page of Queen Isabella, saw in the isle of Guanahani, on the memorable night of the discovery of the New World.
On the 17th, in the morning, the horizon was foggy, and the sky slightly covered with vapour. The outlines of the mountains of Lancerota appeared stronger: the humidity, increasing the transparency of the air, seemed at the same time to have brought the objects nearer our view. This phenomenon is well known to all who have made hygrometrical observations in places whence the chain of the Higher Alps or of the Andes is seen. We passed through the channel which divides the isle of Alegranza from Montana Clara, taking soundings the whole way; and we examined the archipelago of small islands situated northward of Lancerota. In the midst of this archipelago, which is seldom visited by vessels bound for Teneriffe, we were singularly struck with the configuration of the coasts. We thought ourselves transported to the Euganean mountains in the Vicentin, or the banks of the Rhine near Bonn. The form of organized beings varies according to the climate, and it is that extreme variety which renders the study of the geography of plants and animals so attractive; but rocks, more ancient perhaps than the causes which have produced the difference of the climate on the globe, are the same in both hemispheres. The porphyries containing vitreous feldspar and hornblende, the phonolite, the greenstone, the amygdaloids, and the basalt, have forms almost as invariable as simple crystallized substances. In the Canary Islands, and in the mountains of Auvergne, in the Mittelgebirge in Bohemia, in Mexico, and on the banks of the Ganges, the formation of trap is indicated by a symmetrical disposition of the mountains, by truncated cones, sometimes insulated, sometimes grouped, and by elevated plains, both extremities of which are crowned by a conical rising.
The whole western part of Lancerota, of which we had a near view, bears the appearance of a country recently convulsed by volcanic eruptions. Everything is black, parched, and stripped of vegetable mould. We distinguished, with our glasses, stratified basalt in thin and steeply-sloping strata. Several hills resembled the Monte Novo, near Naples, or those hillocks of scoria and ashes which the opening earth threw up in a single night at the foot of the volcano of Jorullo, in Mexico. In fact, the abbe Viera relates, that in 1730, more than half the island changed its appearance. The great volcano, which we have just mentioned, and which the inhabitants call the volcano of Temanfaya, spread desolation over a most fertile and highly cultivated region: nine villages were entirely destroyed by the lavas. This catastrophe had been preceded by a tremendous earthquake, and for several years shocks equally violent were felt. This last phenomenon is so much the more singular, as it seldom happens after an eruption, when the elastic vapours have found vent by the crater, after the ejection of the melted matter. The summit of the great volcano is a rounded hill, but not entirely conic. From the angles of altitude which I took at different distances, its absolute elevation did not appear to exceed three hundred toises. The neighbouring hills, and those of Alegranza and Isla Clara, were scarcely above one hundred or one hundred and twenty toises. We may be surprised at the small elevation of these summits, which, viewed from the sea, wear so majestic a form; but nothing is more uncertain than our judgment on the greatness of angles, which are subtended by objects close to the horizon. From illusions of this sort it arose, that before the measures of Messrs. de Churruca and Galleano, at Cape Pilar, navigators considered the mountains of the straits of Magellan, and those of Terra del Fuego, to be extremely elevated.
The island of Lancerota bore formerly the name of Titeroigotra. On the arrival of the Spaniards, its inhabitants were distinguished from the other Canarians by marks of greater civilization. Their houses were built with freestone, while the Guanches of Teneriffe dwelt in caverns. At Lancerota, a very singular custom prevailed at that time, of which we find no example except among the people of Thibet. A woman had several husbands, who alternately enjoyed the prerogatives due to the head of a family. A husband was considered as such only during a lunar revolution, and whilst his rights were exercised by others, he remained classed among the household domestics. In the fifteenth century the island of Lancerota contained two small distinct states, divided by a wall; a kind of monument which outlives national enmities, and which we find in Scotland, in China, and Peru.
We were forced by the winds to pass between the islands of Alegranza and Montana Clara, and as none on board the sloop had sailed through this passage, we were obliged to be continually sounding. We found from twenty-five to thirty-two fathoms. The lead brought up an organic substance of so singular a structure that we were for a long time doubtful whether it was a zoophyte or a kind of seaweed. The stem, of a brownish colour and three inches long, has circular leaves with lobes, and indented at the edges. The colour of these leaves is a pale green, and they are membranous and streaked like those of the adiantums and Gingko biloba. Their surface is covered with stiff whitish hairs; before their opening they are concave, and enveloped one in the other. We observed no mark of spontaneous motion, no sign of irritability, not even on the application of galvanic electricity. The stem is not woody, but almost of a horny substance, like the stem of the Gorgons. Azote and phosphorus having been abundantly found in several cryptogamous plants, an appeal to chemistry would be useless to determine whether this organized substance belonged to the animal or vegetable kingdom. Its great analogy to several sea-plants, with adiantum leaves, especially the genus caulerpa of M. Lamoureux, of which the Fucus proliter of Forskael is one of the numerous species, engaged us to rank it provisionally among the sea-wracks, and give it the name of Fucus vitifolius. The bristles which cover this plant are found in several other fuci.* (* Fucus lycopodioides, and F. hirsutus.) The leaf, examined with a microscope at the instant we drew it up from the water, did not present, it is true, those conglobate glands, or those opaque points, which the parts of fructification in the genera of ulva and fucus contain; but how often do we find seaweeds in such a state that we cannot yet distinguish any trace of seeds in their transparent parenchyma.
The vine-leaved fucus presents a physiological phenomenon of the greatest interest. Fixed to a piece of madrepore, this seaweed vegetates at the bottom of the ocean, at the depth of 192 feet, notwithstanding which we found its leaves as green as those of our grasses. According to the experiments of Bouguer, light is weakened after a passage of 180 feet in the ratio of 1 to 1477.8. The seaweed of Alegranza consequently presents a new example of plants which vegetate in great obscurity without becoming white. Several germs, enveloped in the bulbs of the lily tribes, the embryo of the malvaceae, of the rhamnoides, of the pistacea, the viscum, and the citrus, the branches of some subterraneous plants; in short, vegetables transported into mines, where the ambient air contains hydrogen or a great quantity of azote, become green without light. From these facts we are inclined to admit that it is not exclusively by the influence of the solar rays that this carburet of hydrogen is formed in the organs of plants, the presence of which makes the parenchyma appear of a lighter or darker green, according as the carbon predominates in the mixture.
Mr. Turner, who has so well made known the family of the seaweeds, as well as many other celebrated botanists, are of opinion that most of the fuci which we gather on the surface of the ocean, and which, from the 23rd to the 35th degree of latitude and 32nd of longitude, appear to the mariner like a vast inundated meadow, grow primitively at the bottom of the ocean, and float only in their ripened state, when torn up by the motion of the waves. If this opinion be well founded, we must agree that the family of seaweeds offers formidable difficulties to naturalists, who persist in thinking that absence of light always produces whiteness; for how can we admit that so many species of ulvaceae and dictyoteae, with stems and green leaves, which float on the ocean, have vegetated on rocks near the surface of the water?
From some notions which the captain of the Pizarro had collected in an old Portuguese itinerary, he thought himself opposite to a small fort, situated north of Teguisa, the capital of the island of Lancerota. Mistaking a rock of basalt for a castle, he saluted it by hoisting the Spanish flag, and sent a boat with an officer to inquire of the commandant whether any English vessels were cruising in the roads. We were not a little surprised to learn that the land which we had considered as a prolongation of the coast of Lancerota, was the small island of Graciosa, and that for several leagues there was not an inhabited place. We took advantage of the boat to survey the land, which enclosed a large bay.
The small part of the island of Graciosa which we traversed, resembles those promontories of lava seen near Naples, between Portici and Torre del Greco. The rocks are naked, with no marks of vegetation, and scarcely any of vegetable soil. A few crustaceous lichen-like variolariae, leprariae, and urceorariae, were scattered about upon the basalts. The lavas which are not covered with volcanic ashes remain for ages without any appearance of vegetation. On the African soil excessive heat and lengthened drought retard the growth of cryptogamous plants.
The basalts of Graciosa are not in columns, but are divided into strata ten or fifteen inches thick. These strata are inclined at an angle of 80 degrees to the north-west. The compact basalt alternates with the strata of porous basalt and marl. The rock does not contain hornblende, but great crystals of foliated olivine, which have a triple cleavage.* (* Blaettriger olivin.) This substance is decomposed with great difficulty. M. Hauy considers it a variety of the pyroxene. The porous basalt, which passes into mandelstein, has oblong cavities from two to eight lines in diameter, lined with chalcedony, enclosing fragments of compact basalt. I did not remark that these cavities had the same direction, or that the porous rock lay on compact strata, as happens in the currents of lava of Etna and Vesuvius. The marl,* (* Mergel.) which alternates more than a hundred times with the basalts, is yellowish, friable by decomposition, very coherent in the inside, and often divided into irregular prisms, analogous to the basaltic prisms. The sun discolours their surface, as it whitens several schists, by reviving a hydro-carburetted principle, which appears to be combined with the earth. The marl of Graciosa contains a great quantity of chalk, and strongly effervesces with nitric acid, even on points where it is found in contact with the basalt. This fact is the more remarkable, as this substance does not fill the fissures of the rock, but its strata are parallel to those of the basalt; whence we may conclude that both fossils are of the same formation, and have a common origin. The phenomenon of a basaltic rock containing masses of indurated marl split into small columns, is also found in the Mittelgebirge, in Bohemia. Visiting those countries in 1792, in company with Mr. Freiesleben, we even recognized in the marl of the Stiefelberg the imprint of a plant nearly resembling the Cerastium, or the Alsine. Are these strata, contained in the trappean mountains, owing to muddy irruptions, or must we consider them as sediments of water, which alternate with volcanic deposits? This last hypothesis seems so much the less admissible, since, from the researches of Sir James Hall on the influence of pressure in fusions, the existence of carbonic acid in substances contained in basalt presents nothing surprising. Several lavas of Vesuvius present similar phenomena. In Lombardy, between Vicenza and Albano, where the calcareous stone of the Jura contains great masses of basalt, I have seen the latter enter into effervescence with the acids wherever it touches the calcareous rock.
We had not time to reach the summit of a hill very remarkable for having its base formed of banks of clay under strata of basalt, like a mountain in Saxony, called the Scheibenbergen Hugel, which is become celebrated on account of the disputes of volcanean and neptunean geologists. These basalts were covered with a mammiform substance, which I vainly sought on the Peak of Teneriffe, and which is known by the names of volcanic glass, glass of Muller, or hyalite: it is the transition from the opal to the chalcedony. We struck off with difficulty some fine specimens, leaving masses that were eight or ten inches square untouched. I never saw in Europe such fine hyalites as I found in the island of Graciosa, and on the rock of porphyry called el Penol de los Banos, on the bank of the lake of Mexico.
Two kinds of sand cover the shore; one is black and basaltic, the other white and quartzose. In a place exposed to the rays of the sun, the first raised the thermometer to 51.2 degrees (41 degrees R.) and the second to 40 degrees (32 degrees R.) The temperature of the air in the shade was 27.7 or 7.5 degrees higher than that of the air over the sea. The quartzose sand contains fragments of feldspar. It is thrown back by the water, and forms, in some sort, on the surface of the rocks, small islets on which seaweed vegetates. Fragments of granite have been observed at Teneriffe; the island of Gomora, from the details furnished me by M. Broussonnet, contains a nucleus of micaceous schist:—the quartz disseminated in the sand, which we found on the shore of Graciosa, is a different substance from the lavas and the trappean porphyries so intimately connected with volcanic productions. From these facts it seems to be evident that in the Canary Islands, as well as on the Andes of Quito, in Auvergne, in Greece, and throughout the greater part of the globe, subterraneous fires have pierced through the rocks of primitive formation. In treating hereafter of the great number of warm springs which we have seen issuing from granite, gneiss, and micaceous schist, we shall have occasion to return to this subject, which is one of the most important of the physical history of the globe.
We re-embarked at sunset, and hoisted sail, but the breeze was too feeble to permit us to continue our course to Teneriffe. The sea was calm; a reddish vapour covered the horizon, and seemed to magnify every object. In this solitude, amidst so many uninhabited islets, we enjoyed for a long time the view of rugged and wild scenery. The black mountains of Graciosa appeared like perpendicular walls five or six hundred feet high. Their shadows, thrown over the surface of the ocean, gave a gloomy aspect to the scenery. Rocks of basalt, emerging from the bosom of the waters, wore the resemblance of the ruins of some vast edifice, and carried our thoughts back to the remote period when submarine volcanoes gave birth to new islands, or rent continents asunder. Every thing which surrounded us seemed to indicate destruction and sterility; but the back-ground of the picture, the coasts of Lancerota presented a more smiling aspect. In a narrow pass between two hills, crowned with scattered tufts of trees, marks of cultivation were visible. The last rays of the sun gilded the corn ready for the sickle. Even the desert is animated wherever we can discover a trace of the industry of man.
We endeavoured to get out of this bay by the pass which separates Alegranza from Montana Clara, and through which we had easily entered to land at the northern point of Graciosa. The wind having fallen, the currents drove us very near a rock, on which the sea broke with violence, and which is noted in the old charts under the name of Hell, or Infierno. As we examined this rock at the distance of two cables' length, we found that it was a mass of lava three or four toises high, full of cavities, and covered with scoriae resembling coke. We may presume that this rock,* (* I must here observe, that this rock is noted on the celebrated Venetian chart of Andrea Bianco, but that the name of Infierno is given, as in the more ancient chart of Picigano, made in 1367, to Teneriffe, without doubt because the Guanches considered the peak as the entrance into hell. In the same latitudes an island made its appearance in 1811.) which modern charts call the West Rock (Roca del Oeste), was raised by volcanic fire; and it might heretofore have been much higher; for the new island of the Azores, which rose from the sea at successive periods, in 1638 and 1719, had reached 354 feet when it totally disappeared in 1723, to the depth of 480 feet. This opinion on the origin of the basaltic mass of the Infierno is confirmed by a phenomenon, which was observed about the middle of the last century in these same latitudes. At the time of the eruption of the volcano of Temanfaya, two pyramidal hills of lithoid lava rose from the bottom of the ocean, and gradually united themselves with the island of Lancerota.
As we were prevented by the fall of the wind, and by the currents, from repassing the channel of Alegranza, we resolved on tacking during the night between the island of Clara and the West Rock. This resolution had nearly proved fatal. A calm is very dangerous near this rock, towards which the current drives with considerable force. We began to feel the effects of this current at midnight. The proximity of the stony masses, which rise perpendicularly above the water, deprived us of the little wind which blew: the sloop no longer obeyed the helm, and we dreaded striking every instant. It is difficult to conceive how a mass of basalt, insulated in the vast expanse of the ocean, can cause so considerable a motion of the waters. These phenomena, worthy the attention of naturalists, are well known to mariners; they are extremely to be dreaded in the Pacific ocean, particularly in the small archipelago of the islands of Galapagos. The difference of temperature which exists between the fluid and the mass of rocks does not explain the direction which these currents take; and how can we admit that the water is engulfed at the base of these rocks, (which often are not of volcanic origin) and that this continual engulfing determines the particles of water to fill up the vacuum that takes place.
The wind having freshened a little towards the morning on the 18th, we succeeded in passing the channel. We drew very near the Infierno the second time, and remarked the large crevices, through which the gaseous fluids probably issued, when this basaltic mass was raised. We lost sight of the small islands of Alegranza, Montana Clara, and Graciosa, which appear never to have been inhabited by the Guanches. They are now visited only for the purpose of gathering archil, which production is, however, less sought after, since so many other lichens of the north of Europe have been found to yield materials proper for dyeing. Montana Clara is noted for its beautiful canary-birds. The note of these birds varies with their flocks, like that of our chaffinches, which often differs in two neighbouring districts. Montana Clara yields pasture for goats, a fact which proves that the interior of this islet is less arid than its coasts. The name of Alegranza is synonymous with the Joyous, (La Joyeuse,) which denomination it received from the first conquerors of the Canary Islands, the two Norman barons, Jean de Bethencourt and Gadifer de Salle. This was the first point on which they landed. After remaining several days at Graciosa, a small part of which we examined, they conceived the project of taking possession of the neighbouring island of Lancerota, where they were welcomed by Guadarfia, sovereign of the Guanches, with the same hospitality that Cortez found in the palace of Montezuma. The shepherd king, who had no other riches than his goats, became the victim of base treachery, like the sultan of Mexico.
We sailed along the coasts of Lancerota, of the island of Lobos, and of Forteventura. The second of these islands seems to have anciently formed part of the two others. This geological hypothesis was started in the seventeenth century by the Franciscan, Juan Galindo. That writer supposed that king Juba had named six Canary Islands only, because, in his time, three among them were contiguous. Without admitting the probability of this hypothesis, some learned geographers have imagined they recognized, in the two islands Nivaria and Ombrios, the Canaria and Capraria of the ancients.
The haziness of the horizon prevented us, during the whole of our passage from Lancerota to Teneriffe, from discovering the summit of the peak of Teyde. If the height of this volcano is 1905 toises, as the last trigonometrical measure of Borda indicates, its summit ought to be visible at a distance of 43 leagues, supposing the eye on a level with the ocean, and a refraction equal to 0.079 of distance. It has been doubted whether the peak has ever been seen from the channel which separates Lancerota from Forteventura, and which is distant from the volcano, according to the chart of Varela, 2 degrees 29 minutes, or nearly 50 leagues. This phenomenon appears nevertheless to have been verified by several officers of the Spanish navy. I had in my hand, on board the Pizarro, a journal, in which it was noted, that the peak of Teneriffe had been seen at 135 miles distance, near the southern cape of Lancerota, called Pichiguera. Its summit was discovered under an angle considerable enough to lead the observer, Don Manual Baruti, to conclude that the volcano might have been visible at nine miles farther. It was in September, towards evening, and in very damp weather. Reckoning fifteen feet for the elevation of the eye, I find, that to render an account of this phenomenon, we must suppose a refraction equal to 0.158 of the arch, which is not very extraordinary for the temperate zone. According to the observations of General Roy, the refractions vary in England from one-twentieth to one-third; and if it be true that they reach these extreme limits on the coast of Africa, (which I much doubt,) the peak, in certain circumstances, may be seen on the deck of a vessel as far off as 61 leagues.
Navigators who have much frequented these latitudes, and who can reflect on the physical causes of the phenomena, are surprised that the peaks of Teyde and of the Azores* (* The height of this peak of the Azores, according to Fleurieu, is 1100 toises; to Ferrer, 1238 toises; and to Tofino, 1260 toises: but these measures are only approximative estimates. The captain of the Pizarro, Don Manuel Cagigal, proved to me, by his journal, that he observed the peak of the Azores at the distance of 37 leagues, when he was sure of his latitude within two minutes. The volcano was seen at 4 degrees south-east, so that the error in longitude must have an almost imperceptible influence in the estimation of the distance. Nevertheless, the angle which the peak of the Azores subtended was so great, that the captain of the Pizarro was of opinion this volcano must be visible at more than 40 or 42 leagues. The distance of 37 leagues supposes an elevation of 1431 toises.) are sometimes visible at a very great distance, though at other times they are not seen when the distance is much less, and the sky appears serene and the horizon free from fogs. These circumstances are the more worthy of attention because vessels returning to Europe, sometimes wait impatiently for a sight of these mountains, to rectify their longitude; and think themselves much farther off than they really are, when in fine weather these peaks are not perceptible at distances where the angles subtended must be very considerable. The constitution of the atmosphere has a great influence on the visibility of distant objects. It may be admitted, that in general the peak of Teneriffe is seldom seen at a great distance, in the warm and dry months of July and August; and that, on the contrary, it is seen at very extraordinary distances in the months of January and February, when the sky is slightly clouded, and immediately after a heavy rain, or a few hours before it falls. It appears that the transparency of the air is prodigiously increased, as we have already observed, when a certain quantity of water is uniformly diffused through the atmosphere. Independent of these observations, it is not astonishing, that the peak of Teyde should be seldomer visible at a very remote distance, than the summits of the Andes, to which, during so long a time, my observations were directed. This peak, inferior in height to those parts of the chain of Mount Atlas at the foot of which is the city of Morocco, is not, like those points, covered with perpetual snows. The Piton, or Sugar-loaf, which terminates the peak, no doubt reflects a great quantity of light, owing to the whitish colour of the pumice-stone thrown up by the crater; but the height of that little truncated cone does not form a twenty-second part of the total elevation. The flanks of the volcano are covered either with blocks of black and scorified lava, or with a luxuriant vegetation, the masses of which reflect the less light, as the leaves of the trees are separated from each other by shadows of more considerable extent than that of the part enlightened.
Hence it results that, setting aside the Piton, the peak of Teyde belongs to that class of mountains, which, according to the expression of Bouger, are seen at considerable distances only in a NEGATIVE MANNER, because they intercept the light which is transmitted to us from the extreme limits of the atmosphere; and we perceive their existence only on account of the difference of intensity subsisting between the aerial light which surrounds them, and that which is reflected by the particles of air placed between the mountains and the eye of the observer. As we withdraw from the isle of Teneriffe, the Piton or Sugar-loaf is seen for a considerable space of time in a POSITIVE MANNER, because it reflects a whitish light, and clearly detaches itself from the sky. But as this cone is only 80 toises high, by 40 in breadth at its summit, it has recently been a question whether, from the diminutiveness of its mass, it can be visible at distances which exceed 40 leagues; and whether it be not probable, that navigators distinguish the peaks as a small cloud above the horizon, only when the base of the Piton begins to be visible on it. If we admit, that the mean breadth of the Sugar-loaf is 100 toises, we find that the little cone, at 40 leagues distance, still subtends, in the horizontal direction, an angle of more than three minutes. This angle is considerable enough to render an object visible; and if the height of the Piton greatly exceeded its base, the angle in the horizontal direction might be still smaller, and the object still continue to make an impression on our visual organs; for micrometrical observations have proved that the limit of vision is but a minute only, when the dimensions of the objects are the same in every direction. We distinguish at a distance, by the eye only, trunks of trees insulated in a vast plain, though the subtended angle be under twenty-five seconds.
As the visibility of an object detaching itself in a brown colour, depends on the quantities of light which the eye meets on two lines, one of which ends at the mountain, and the other extends to the surface of the aerial ocean, it follows that the farther we remove from the object, the smaller the difference becomes between the light of the surrounding atmosphere, and that of the strata of air before the mountain. For this reason, when less elevated summits begin to appear above the horizon, they present themselves at first under a darker hue than those we discern at very great distances. In the same manner, the visibility of mountains seen only in a negative manner, does not depend solely on the state of the lower regions of the air, to which our meteorological observations are limited, but also on the transparency and physical constitution of the air in the most elevated parts; for the image detaches itself better in proportion as the aerial light, which comes from the limits of the atmosphere, has been originally more intense, or has undergone less loss in its passage. This consideration explains to a certain point, why, under a perfectly serene sky, the state of the thermometer and the hygrometer being precisely the same in the air nearest the earth, the peak is sometimes visible, and at other times invisible, to navigators at equal distances. It is even probable, that the chance of perceiving this volcano would not be greater, if the ashy cone, at the summit of which is the mouth of the crater, were equal, as in Vesuvius, to a quarter of the total height. These ashes, being pumice-stone crumbled into dust, do not reflect as much light as the snow of the Andes; and they cause the mountain, seen from afar, to detach itself not in a bright, but in a dark hue. The ashes also contribute, if we may use the expression, to equalize the portions of aerial light, the variable difference of which renders the object more or less distinctly visible. Calcareous mountains, devoid of vegetable earth, summits covered with granitic sand, the high savannahs of the Cordilleras,* (* Los Pajonales, from paja, straw. This is the name given to the region of the gramina, which encircles the zone of the perpetual snows.) which are of a golden yellow, are undoubtedly distinguished at small distances better than objects which are seen in a negative manner; but the theory indicates a certain limit, beyond which these last detach themselves more distinctly from the azure vault of the sky.
The colossal summits of Quito and Peru, towering above the limit of the perpetual snows, concentre all the peculiarities which must render them visible at very small angles. The circular summit of the peak of Teneriffe is only a hundred toises in diameter. According to the measures I made at Riobamba, in 1803, the dome of the Chimborazo, 153 toises below its summit, consequently in a point which is 1300 toises higher than the peak, is still 673 toises (1312 metres) in breadth. The zone of perpetual snows also forms a fourth of the height of the mountain; and the base of this zone, seen on the coast of the Pacific, fills an extent of 3437 toises (6700 metres). But though Chimborazo is two-thirds higher than the peak, we do not see it, on account of the curve of the globe, at more than 38 miles and a third farther distant. The radiant brilliancy of its snows, when, at the port of Guayaquil, at the close of the rainy season, Chimborazo is discerned on the horizon, may lead us to suppose, that it must be seen at a very great distance in the South Sea. Pilots highly worthy of credit have assured me, that they have seen it from the rock of Muerto, to the south west of the isle of Puna, at a distance of 47 leagues. Whenever it has been seen at a greater distance, the observers, uncertain of their longitude, have not been in a situation to furnish precise data.
Aerial light, projected on mountains, increases the visibility of those which are seen positively; its power diminishes, on the contrary, the visibility of objects which, like the peak of Teneriffe and that of the Azores, detach themselves in a brown tint. Bouguer, relying on theoretical considerations, was of opinion that, according to the constitution of our atmosphere, mountains seen negatively cannot be perceived at distances exceeding 35 leagues. It is important here to observe, that these calculations are contrary to experience. The peak of Teneriffe has been often seen at the distance of 36, 38, and even at 40 leagues. Moreover, in the vicinity of the Sandwich Islands, the summit of Mowna-Roa, at a season when it was without snows, has been seen on the skirt of the horizon, at the distance of 53 leagues. This is the most striking example we have hitherto known of the visibility of a mountain; and it is the more remarkable, that an object seen negatively furnishes this example.
The volcanoes of Teneriffe, and of the Azores, the Sierra Nevada of Santa Martha, the peak of Orizaba, the Silla of Caracas, Mowna-Roa, and Mount St. Elias, insulated in the vast extent of the seas, or placed on the coasts of continents, serve as sea-marks to direct the pilot, when he has no means of determining the position of the vessel by the observation of the stars; everything which has a relation to the visibility of these natural seamarks, is interesting to the safety of navigation.
