32

DISTANCE AT WHICH MOUNTAINS

vested with perpetual snow. The Sugar-loaf which constitutes the summit of the former no doubt reflects a great degree of light, on account of the white colour of the pumice with which it is covered; but its height does not form a twentieth part of the total elevation, and the sides of the volcano are coated with blocks of dark-coloured lava, or with luxuriant vegetation, the masses of which reflect little light, the leaves of the trees being separated by shadows of greater extent than the illuminated parts.

Hence the Peak of Teneriffe is to be referred to the class of mountains which are seen at great distances only in what Bouguer calls a negative manner, or because they intercept the light transmitted from the extreme limits of the atmosphere; and we perceive their existence only by means of the difference of intensity that subsists between the light which surrounds them, and that reflected by the particles of air placed between the object of vision and the observer. In receding from Teneriffe, the Sugarloaf is long seen in a positive manner, as it reflects a whitish light, and detaches itself clearly from the sky; but as this terminal cone is only 512 feet high, by 256 in breadth at its summit, it has been questioned whether it can be visible beyond the distance of 138 miles. If it be admitted that the mean breadth of the Sugar-loaf is 639 feet, it will still subtend, at the distance now named, an angle of more than three minutes, which is enough to render it visible; and were the height of the cone greatly to exceed its basis, the angle might be still less, and the mass yet make an impression on our organs; for it has been proved by micrometrical observations, that the limit of vision is one minute only when the dimensions of objects are the same in all directions.

As the visibility of an object, which detaches itself from the sky of a brown colour, depends on the quantities of light the eye meets in two lines, of which one ends at the mountain and the other is

MAY BE SEEN AT SEA.

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prolonged to the surface of the aerial ocean, it follows that the farther we remove from the object the less also becomes the difference between the light of the surrounding atmosphere and that of the strata of air placed before the mountain. For this reason, when summits of low elevation begin to appear above the horizon, they are of a darker tint than those more elevated ones which we discover at

very great distances. In like manner, the visibility of mountains which are only negatively perceived does not depend solely upon the state of the low regions of the air, to which our meteorological observations are confined, but also upon its transparency and physical constitution in the most elevated parts; for the image is more distinctly detached, the more intense the aerial light which comes from the limits of the atmosphere has originally been, or the less it has lost in its passage. This in a certain degree accounts for the circumstance that the Peak is sometimes visible and sometimes invisible to navigators who are equally distant from it, when the state of the thermometer and hygrometer is precisely the same in the lower stratum of air. It is even probable that the chance of perceiving this volcano would not be greater were the cone equal, as in Vesuvius, to a fourth part of the whole height. The ashes spread upon its surface do not reflect so much light as the snow with which the summits of the Andes are covered; but, on the contrary, make the mountain, when seen from a great distance, become more obscurely detached, and assume a brown tint. They contribute, as it were, to equalize the portions of aerial light, the variable difference of which renders the object more or less distinctly visible. Bare calcareous mountains, summits covered with granitic sand, and the elevated savannas of the Andes, which are of a bright yellow colour, are more clearly seen at small distances than objects that are perceived only in a negative manner; but theory

points out a limit beyond which the latter are more distinctly detached from the azure vault of the sky.

The aerial light projected on the tops of hills increases the visibility of those which are seen positively, but diminishes that of such as are detached with a brown colour. Bouguer, proceeding on theoretical data, has found that mountains which are seen negatively cannot be perceived at distances exceeding 121 miles; but experience goes against this conclusion. The Peak of Teneriffe has often been observed at the distance of 124, 131, and even 138 miles; and the summit of Mowna-Roa in the Sandwich Isles, which is probably 16,000 feet high, has been seen, at a period when it was destitute of snow, skirting the horizon from a distance of 183 miles. This is the most striking example yet known of the visibility of high land, and is the more remarkable that the object was negatively seen.

The atmosphere continuing hazy, the navigators did not discover the island of Grand Canary, notwithstanding its height, until the evening of the 18th June. On the following day they saw the point of Naga, but the Peak of Teneriffe still remained invisible. After repeatedly sounding, on account of the thickness of the mist, they anchored in the road of Santa Cruz, when at the moment they began to salute the place the fog instantaneously dispersed, and the Peak of Teyde, illuminated by the first rays of the sun, appeared in a break above the clouds. Our travellers betook themselves to the bow of the vessel to enjoy the majestic spectacle, when, at the very moment, four English ships were seen close astern. The anchor was immediately got up, and the Pizarro stood in as close as possible, to place herself under the protection of the fort.

While waiting the governor's permission to land, Humboldt employed the time in making observations for determining the longitude of the mole of Santa Cruz and the dip of the needle. Berthoud's chro

SANTA CRUZ OF TENERIFFE.

35

nometer gave 18° 33' 10", the accuracy of which result, although differing from the longitude assigned by Cook and others, was afterward confirmed by Krusenstern, who found that port 16° 12′ 45′′ west of Greenwich, and consequently 18° 33' west of Paris. The dip of the magnetic needle was 62° 24', although it varied considerably in different places along the shore. After undergoing the fatigue of answering the numberless questions proposed by persons who visited them on board, our travellers were at length permitted to land.

CHAPTER III.

Island of Teneriffe.

Santa Cruz-Villa de la Laguna--Guanches-Present Inhabitants of Teneriffe-Climate-Scenery of the Coast-Orotava-Dragon-treeAscent of the Peak-Its Geological Character--Eruptions--Zones of Vegetation-Fires of St. John.

SANTA CRUZ, the Anaja of the Guanches, which is a neat town, with a population of 8000 persons, may be considered as a great caravansera situated on the road to America and India, and has consequently been often described. The recommendations of the court of Madrid procured for our travellers the most satisfactory reception in the Canaries. The captain-general gave permission to examine the island, and Colonel Armiaga, who commanded a regiment of infantry, extended his hospitality to them, and showed the most polite attention. In his garden they admired the banana, the papaw, and other plants cultivated in the open air, which they had before seen only in hothouses.

In the evening they made a botanical excursion

36

VILLA DE LA LAGUNA.

towards the fort of Passo Alto, along the basaltic rocks which close the promontory of Naga, but had little success, as the drought and dust had in a manner destroyed the vegetation. The Cacalia kleinia, Euphorbia canariensis, and other succulent plants, which derive their nourishment more from the air than from the soil, reminded them by their aspect that the Canaries belong to Africa, and even to the most arid part of that continent.

The captain of the Pizarro, having apprized them that, on account of the blockade by the English, they ought not to reckon upon a longer stay than four or five days, they hastened to set out for the port of Orotava, where they might find guides for the ascent of the Peak; and on the 20th, before sunrise, they were on the way to Villa de la Laguna, which is 2238 feet higher than the port of Santa Cruz. The road to this place is on the right of a torrent, which, in the rainy season, forms beautiful falls. Near the town they met with some white camels, employed in transporting merchandise. These animals, as well as horses, were introduced into the Canary Islands in the fifteenth century by the Norman conquerors, and were unknown to the Guanches. Camels are more abundant in Lancerota and Forteventura, which are nearer the continent, than at Teneriffe, where they very seldom propagate.

The hill on which the Villa de la Laguna stands belongs to the series of basaltic mountains which forms a girdle around the Peak, and is independent of the newer volcanic rocks. The basalt on which the travellers walked was blackish-brown, compact, and partially decomposed. They found in it hornblende, olivine, and transparent pyroxene, with lamellar fracture, of an olive-green tint, and often crystallized in six-sided prisms. The rock of Laguna is not columnar, but divided into thin beds, inclined at an angle of from 30° to 48°, and has no