Note 7 — ON STEPPES AND DESERTS.

p. 3—“When suddenly deserted by the tropical east wind, and the sea is covered with weeds.”

It is a remarkable phenomenon, although one generally known to mariners, that in the neighbourhood of the African coast, (between the Canaries and the Cape de Verde islands, and more especially between Cape Bojador and the mouth of the Senegal,) a westerly wind often prevails instead of the usual east or trade wind of the tropics. The cause of this phenomenon is to be ascribed to the far-extending desert of Zahara, and arises from the rarefaction, and consequent vertical ascent of the air over the heated sandy surface. To fill up the vacuum thus occasioned, the cool sea-air rushes in, producing a westerly breeze, adverse to vessels sailing to America; and the mariner, long before he perceives any continent, is made sensible of the effects of its heat-radiating sands. As is well known, a similar cause produces that alternation of sea and land breezes, which prevails at certain hours of the day and night on all sea-coasts.

The accumulation of sea-weed in the neighbourhood of the western coasts of Africa has been often referred to by ancient writers. The local position of this accumulation is a problem which is intimately connected with the conjectures regarding the extent of Phœnician navigation. The Periplus, which has been ascribed to Scylax of Caryanda, and which, according to the investigations of Niebuhr and Letronne, was very probably compiled in the time of Philip of Macedon, contains a description of a kind of fucus sea, Mar de Sargasso, beyond Cerne; but the locality indicated appears to me very different from that assigned to it in the work “De Mirabilibus Auscultationibus,” which for a long time, but incorrectly, bore the great name of Aristotle.^[AK] “Driven by the east wind,” says the pseudo-Aristotle, “Phœnician 47mariners came in a four days’ voyage from Gades to a place where the sea was found covered with rushes and sea-weed (θρύον καὶ φῦκος). The sea-weed is uncovered at ebb, and overflowed at flood tide.” Does he not here refer to a shoal lying between the 34th and 36th degrees of latitude? Has a shoal disappeared there in consequence of volcanic revolution? Vobonne refers to rocks north of Madeira.^[AL] In Scylax it is stated that “the sea beyond Cerne ceases to be navigable in consequence of its great shallowness, its muddiness, and its sea-grass. The sea-grass lies a span thick, and it is pointed at its upper extremity, so that it pricks.” The sea-weed which is found between Cerne (the Phœnician station for merchant vessels, Gaulea; or, according to Gosselin, the small estuary of Fedallah, on the north-west coast of Mauritania,) and Cape Verde, at the present time by no means forms a great meadow or connected group, “mare herbidum,” such as exists on the other side of the Azores. Moreover, in the poetic description of the coast given by Festus Avienus,^[AM] in which, as Avienus himself very distinctly acknowledges, he availed himself of the journals of Phœnician ships, the impediments presented by the sea-weed are described with great minuteness; but Avienus places the site of this obstacle much further north, towards Ierne, the Holy Isle.

Sic nulla late flabra propellunt ratem,

Sic segnis humor æquoris pigri stupet.

Adjicit et illud, plurimum inter gurgites

Exstare fucum, et sæpe virgulti vice

Retinere puppim ...

Hæc inter undas multa cæspitem jacet,

Eamque late gens Hibemorum colit.

When we consider that the sea-weed (fucus), the mud or slime (πηλὸς), the shallowness of the sea, and the perpetual calms, are always regarded by the ancients as characteristic of the Western Ocean beyond the Pillars of Hercules, we feel inclined, especially on account of the reference to the calms, to ascribe this to Punic cunning, to the tendency of a great trading people to hinder others, by terrific descriptions, from competing with them in maritime trading westwards. But even 48in the genuine writings of the Stagyrite,^[AN] the same opinion is retained regarding the absence of wind, and Aristotle attempts to explain a false notion, or, as it seems to me, more correctly speaking, a fabulous mariner’s story, by an hypothesis regarding the depth of the sea. The stormy sea between Gades and the Islands of the Blest (Cadiz and the Canaries) can in truth in no way be compared with the sea, which lies between the tropics, ruffled only by the gentle trade-winds (vents alisés), and which has been very characteristically named by the Spaniards^[AO] El Golfo de las Damas.

From very careful personal researches and from comparison of the logs of many English and French vessels, I am led to believe that the old and very indefinite expression Mar de Sargasso, refers to two fucus banks, the larger of which is of an elongated form, and is the easternmost one, lying between the parallels of 19° and 34°, in a meridian 7° westward of the Island of Corvo, one of the Azores; while the smaller and westernmost bank is of a roundish form, and is found between Bermuda and the Bahama Islands (lat. 25°–31°, long. 66°–74°). The principal diameter of the small bank, which is traversed by ships sailing from Baxo de Plata (Caye d’Argent,) northward of St. Domingo to the Bermudas, appears to me to have a N. 60° E. direction. A transverse band of fucus natans, extending in an east-westerly direction between the latitudes of 25° and 30°, connects the greater with the smaller bank. I have had the pleasure of seeing these views adopted by my lamented friend Major Rennell, and confirmed, in his great work on Currents, by many new observations.^[AP] The two groups of sea-weed, together with the transverse band uniting them, constitute the Sargasso Sea of the older writers, and collectively occupy an area equal to six or seven times that of Germany.

The vegetation of the ocean thus offers the most remarkable example of social plants of a single species. On the main land the Savannahs or grass plains of America, the heaths (ericeta), and the forests of Northern Europe and Asia, 49in which are associated coniferous trees, birches, and willows, produce a less striking uniformity than do these thalassophytes. Our heaths present in the north not only the predominating Calluna vulgaris, but also Erica tetralix, E. ciliaris, and E. cinerea; and in the south, Erica arborea, E. scoparia, and E. Mediterranea. The uniformity of the view presented by the Fucus natans is incomparably greater than that of any other assemblage of social plants. Oviedo calls the fucus banks “meadows,” praderias de yerva. If we consider that Pedro Velasco, a native of the Spanish harbour of Palos, by following the flight of certain birds from Fayal, discovered the Island of Flores as early as 1452, it seems almost impossible, considering the proximity of the great fucus bank of Corvo and Flores, that no part of these oceanic meadows should have been seen before the time of Columbus by Portuguese ships driven westward by storms.

We learn, however, from the astonishment of the companions of the admiral, when they were continuously surrounded by sea-grass from the 16th of September to the 8th of October, 1492, that the magnitude of the phenomenon was at that period unknown to mariners. In the extracts from the ship’s journal given by Las Casas, Columbus certainly does not mention the apprehensions which the accumulation of sea-weed excited, or the grumbling of his companions. He merely speaks of the complaints and murmurs regarding the danger of the very weak but constant east winds. It was only his son, Fernando Colon, who in the history of his father’s life, endeavoured to give a somewhat dramatic delineation of the anxieties of the sailors.

According to my researches, Columbus made his way through the great fucus bank in the year 1492, in latitude 28½°, and in 1493, in latitude 37°, and both times in the longitude of 38°–41°. This can be established with tolerable certainty from the estimation of the velocity recorded by Columbus, and “the distance daily sailed over;” not indeed by dropping the log, but by the information afforded by the running out of half-hour sand-glasses (ampolletas). The first certain and distinct account of the log, (catena della poppa,) which I have found, is in the year 1521, in Pigafetta’s Journal of Magellan’s Circumnavigation of the World.^[AQ] The determination 50of the ship’s place during the days in which Columbus was crossing the great bank is the more important, because it shews us that for three centuries and a half the total accumulation of these socially-living thalassophytes, (whether consequent on the local character of the sea’s bottom or on the direction of the recurrent Gulf stream,) has remained at the same point. Such evidences of the persistence of great natural phenomena doubly arrest the attention of the natural philosopher, when they occur in the ever-moving oceanic element. Although the limits of the fucus banks oscillate considerably, in accordance with the strength and direction of long predominating winds, yet we may still, in the middle of the nineteenth century, take the meridian of 41° west of Paris (or 8° 38′ west of Greenwich) as the principal axis of the great bank. Columbus, with his vivid imaginative force, associated the idea of the position of this bank with the great physical line of demarcation, which according to him, “separated the globe into two parts, and was intimately connected with the changes of magnetic deviation and of climatic relations.” Columbus when he was uncertain regarding the longitude, attempted to determine his place (February, 1493,) by the appearance of the first floating masses of tangled weed (de la primera yerva) on the eastern border of the great Corvo bank. The physical line of demarcation was, by the powerful influence of the Admiral, converted on the 4th of May, 1493, into a political one, in the celebrated line of demarcation between the Spanish and Portuguese rights of possession^[AR].