Roa*, at a season when it was without snows, was 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 what is the more remarkable, it is an object seen negatively which furnishes the example.

I thought it proper to bring together these considerations at the end of this chapter, because in treating so closely on one of the most important problems of optics, that of the diminution

* The height of Mowna Roa, according to Marchand, is more than 2598 toises; according to King, it is 2577 toises; but these measures, notwithstanding their accidental concordance, are not founded on very exact measurements. It is a very extraordinary phenomenon, to see a summit placed in the 19th degree of latitude, and which is probably 2500 toises high, entirely stripped of it's snows. The very flattened form of Mowna Roa, the Mesa of the old Spanish charts, it's insulated situation in the midst of the ocean, and the frequency of certain winds, which, modified by the ascending current, blow obliquely, may be the principal causes. It is difficult to believe, that captain Marchand was much deceived in the estimation of the distance at which he saw, on the 10th of October 1791, the summit of Mowna Roa. He had left the island of Owhyhee only the 7th in the evening; and from the movement of the waters, and the lunar observations of the 19th, it is probable that the distance was even greater than 53 leagues. Besides, an experienced navigator, Mr. Fleurieu, relates, that at a distance of 35 or 36 leagues the peak of Teneriffe is visible, even in weather that is not perfectly clear. Voy. de Marchand, vol. i, p. 408 and 427 ; vol. ii, p: 10 and 78.

of light in it's passage across the strata of the atmosphere, they may be at the same time of some practical utility. The volcanoes of Teneriffe, and of the Azores, the Sierra Nevada of St. Martha, the peak of Orizaba, the Silla of Caraccas, 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, who is deprived of the means fitted to determine the position of the vessel by the observation of the stars; every thing, which has a relation to the visibility of these natural sea marks, is interesting to the safety of navigation.

CHAPTER II.

Stay at Teneriffe.-Journey from Santa Cruz to Orotava.--Excursion to the top of the Peak of Teyde.

FROM the time of our departure from Graciosa, the horizon continued so hazy, that notwithstanding the considerable height of the mountains of Canary*, we did not discover this island till the evening of the 18th of June. It is the granary of the archipelago of the Fortunate islands, and what is very remarkable in a region situate beyond the limits of the tropics, we were assured, that in some districts, there are two wheat harvests in the year; one in February, and the other in June. Canary has never been visited by a learned mineralogist; yet this island is so much the more worthy of observation, as the physiognomy of it's mountains, disposed in parallel chains, appeared to me to differ entirely from that of the summits of Lanzerota and Te

*Isla de la Gran Canaria.

+ Ledra, Voyage à Teneriffe, t. i, p. 37.

neriffe. Nothing is more interesting for the geologist, than to observe the relations, on the same point of the globe, between volcanic countries, and those which are primitive or secondary. When the Canary islands shall have been some day examined in all the parts, which compose the system of these mountains, we shall find, that we have been too precipitate in considering the whole group as raised by the action of submarine fires.

The 19th, in the morning, we discovered the point of Naga*, but the Peak of Teneriffe was still invisible: the land, obscured by a thick fog, presented forms that were vague and confused. As we approached the road of Santa Cruz, we observed that these vapours, driven by the winds, drew nearer. The sea was strongly agitated, as it most commonly is in those latitudes. We anchored after several soundings, for the mist was so thick, that we could scarcely distinguish objects at a few cables' distance; but at the moment we began to salute the place, the fog was instantly dispelled. The peak of Teyde appeared in a break above the clouds, and the first rays of the sun, which had not yet risen on us, illuminated the summit of the volcano. We hastened toward the bow of the vessel, to enjoy the magnificent spectacle, when at the same instant we

* Punta de Naga, Anaga, or Nago.

saw four English ships of the royal navy lying to, very near the poop. We had passed without being perceived; and the same mist which had hidden the peak from our view, had saved us from the danger of being carried back to Europe. It would have been very painful to naturalists, to have seen the coast of Teneriffe, without having been able to tread a soil torn up by volcanoes.

We immediately got up our anchor, and the Pizarro stood in as close as possible to the fort, to be under it's protection. It was on this shore, that, in the landing attempted by the English two years before our arrival*, admiral Nelson had his arm carried off by a cannon ball. The governor general of the Canaries+ sent an order to the captain of the sloop, to put on shore the dispatches from the court for the governors of the colonies, the money on board, and the public correspondence. The English vessels left the road, having given chace the evening before to the packet boat the Alcudia, which had left Corunna a few days before us. She was obliged to touch at the port of Palmas, in the isle of Canary, and several passengers, who were going in a boat to Santa Cruz, had been made prisoners.

The situation of this town is very similar to that of La Guayra, the most frequented port of

VOL. I.

* In the month of July, 1797.

+ Don Andrea de Perlasca.

I