Note 7 — STEPPES AND DESERTS.

p. 3.—“When suddenly deserted by the east wind of the tropics in a sea covered with weed.”

It is a remarkable phenomenon, well known among sailors, that in the vicinity of the African coast (between the Canaries and the Cape de Verde Islands, and particularly between Cape Bojador and the mouth of the Senegal), a west wind often takes the place of the general east or trade-wind of the tropics. It is the wide expanse of the desert of Sahara which causes this westerly wind. The air over the heated sandy plain becomes rarefied and ascends, the air from the sea rushes in to supply the void so formed, and thus there sometimes arises a west wind, adverse to ships bound to the American coast, which are made in this manner to feel the vicinity of the heat-radiating desert without even seeing the continent to which it belongs. The changes of land and sea breezes, which blow alternately at certain hours of the day or night on all coasts, are due to the same causes.

The accumulation of sea-weed in the neighbourhood of the African coast has been often spoken of by ancient writers. The locality of this accumulation is a problem which is intimately connected with our conjectures respecting the extent of Phœnician navigation. The Periplus, which has been ascribed to Scylax of Caryanda, and which, according to the researches of Niebuhr and Letronne, was very probably compiled in the time of Philip of Macedon, describes beyond Cerne a quantity of fucus forming a weed-covered sea—a kind of “Mar de Sargasso;” but the locality indicated appears to me to differ very much from that assigned in the work entitled “De Mirabilibus Auscultationibus,” which long bore, unduly, the great name of Aristotle. (Compare Scyl. Caryand. Peripl. in Hudson, vol. ii. p. 53, with Aristot. de Mirab. Auscult. in opp. omnia ex. rec. Bekkeri, p. 844, § 136.) The pseudo-Aristotle says, “Phœnician mariners, driven by the east wind, came in four days’ sail from Gades to a part where they found the sea covered with reeds and sea-weed (θρύον καὶ φῦκος.) The sea-weed is uncovered at ebb and covered at flood tide.” Is he not here speaking of a shallow place between the 34° and 36° of latitude? Has a shoal disappeared in consequence of volcanic eruption? Vobonne speaks of rocks north of Madeira. (Compare also Edrisi, Geog. Nub., 1619, p. 157.) In Scylax it is said, “The sea beyond Cerne is unnavigable on account of its great shallowness, its muddiness, and the great quantity of sea grasses. The sea grass lies a span thick, and is full of points at the top, so that it pricks.” The sea-weed found between Cerne,—(the Phœnician station for laden vessels, Gaulea, or, according to Gosselin, the small island of Fedallah, on the north-western coast of Mauritania),—and Cape de Verde, does not now by any means form a great sea meadow, or connected tract of fucus, a “mare herbidum,” such as exists beyond the Azores. In the poetic description of the coast by Festus Avienus, (Ora Maritima, v. 109, 122, 388, and 408), in the composition of which, as Avienus himself says, (v. 412) he availed himself of the journals of Phœnician ships, the obstacle presented by the sea-weed is referred to in a very circumstantial manner; but its site is placed much farther north, towards Ierne, the “Sacred Island.”

In remarking that the fucus and the mud or mire, (πηλός), the shallowness of the sea, and the perpetual calms, are always spoken of by the ancients as characteristics of the western ocean beyond the Pillars of Hercules, one is disposed, more particularly on account of the mention of the calms, to ascribe something to Punic artifice,—to the desire of a great trading people to deter others, by the apprehension of dangers and difficulties, from entering into competition with them in western navigation and commerce. But even in the genuine writings of Aristotle (Meteorol. ii. p. 1, 14,) he maintains this same opinion of the absence of wind in those regions, and seeks the explanation of what he erroneously supposes to be a fact of observation, but which is more properly a fabulous mariner’s tale, in an hypothesis concerning the depth of the sea. In reality, the stormy sea between Gades and the islands of the Blest or Fortunate Islands, (between Cadix and the Canaries), is very unlike the sea farther to the south between the tropics, where the gentle trade winds blow, and which is called very characteristically by the Spaniards, el Golfo de las Damas, the Ladies’ Gulf. (Acosta Historia natural y moral de las Indias, lib. iii. cap. 4.)

From very careful researches by myself, and from the comparison of the logs or journals of many English and French vessels, I infer that the old and indefinite expression, Mar de Sargasso, includes two banks of fucus, of which the greater and easternmost one, of a lengthened shape, is situated between the parallels of 19° and 34° N. lat., in a meridian of 7 degrees to the west of the Island of Corvo, one of the Azores; while the lesser and westernmost bank, of a roundish form, is situated between the Bermudas and the Bahamas, (lat. 25°-31°, long. 66°-74°.) The longer axis of the small bank which is crossed by ships going from Baxo de Plata (Caye d’Argent, Silver Cay) on the north of St. Domingo, to the Bermudas, appears to have a N. 60° E. direction. A transverse band of Fucus natans, running in an East and West direction between the parallels of 25° and 30°, connects the greater and lesser banks. I have had the gratification of seeing these inferences approved by my honoured friend Major Rennell, and adopted by him in his great work on Currents, where he has further supported and confirmed them by many new and additional observations. (Compare Humboldt, Relation Historique, tom. i. p. 202, and Examen critique, tom. iii. p. 68–99, with Rennell’s Investigation of the Currents of the Atlantic Ocean, 1832, p. 184.) The two groups of sea-weed, included together with the transverse connecting band under the old general name of the Sargasso Sea, occupy altogether a space exceeding six or seven times the area of Germany.

Thus it is the vegetation of the ocean which offers the most remarkable example of an assemblage of “social plants” of a single species. On terra firma, the savannahs or prairies, or grassy plains of America, the heaths (ericeta), and the forests of the North of Europe and Asia, consisting of coniferous trees, birches, and willows, offer a less degree of uniformity than do those thalassophytes. Our heaths show, in the north, in addition to the prevailing Calluna vulgaris, Erica tetralix, E. ciliaris, and E. cinerea; and in the south, Erica arborea, E. scoparia, and E. mediterranea. The uniformity of the aspect offered by the Fucus natans is greater than that of any other assemblage or association of plants. Oviedo calls the fucus banks “meadows,” praderias de yerva. Considering that the island of Flores was discovered in 1452, by Pedro Velasco, a native of the Spanish port of Palos, by following the flight of certain birds from the island of Fayal, it seems almost impossible, seeing the proximity of the great fucus bank of Corvo and Flores, that a part of these oceanic meadows should not have been seen before Columbus, by Portuguese ships driven by storms to the westward. Yet the astonishment of the companions of Columbus in 1492, when surrounded by sea-weed uninterruptedly from the 16th of September to the 8th of October, shews that the magnitude of the phenomenon at least was previously unknown to the sailors. The anxieties excited by the accumulation of sea-weed, and the murmurs of his companions in reference thereto, are not indeed mentioned by Columbus in the extracts from the ship’s journal given by Las Casas. He merely speaks of the complaints and murmurs respecting the danger to be feared from the weak but constant East winds. It is only the son, Fernando Colon, who, in writing his father’s life, endeavoured to depict the fears of the sailors in a dramatic manner.

According to my researches, Columbus crossed the great fucus bank in 1492, in lat. 28½°, and in 1493, in lat. 37°, both times in the long. of from 38° to 41° W. This is deducible with tolerable certainty from Columbus’s recorded estimation of the ship’s rate, and the “distance daily sailed over;” derived indeed, not from casting the log, but from data afforded by the running out of half-hour sand-glasses (ampolletas). The first certain and definite mention of a log (catena della poppa) which I have been able to discover, is in the year 1521, in Pigafetta’s journal of Magellan’s Voyage round the World. (Cosmos, vol. ii. p. 259, and Note 405, English ed.) The determination of the ship’s place, while Columbus was engaged in traversing the great meadows of sea-weed, is the more important, because we learn from it that for three centuries and a half the situation of this great accumulation of thalassophytes, whether resulting from the local character of the bottom of the sea, or from the direction of the Gulf stream, has remained the same. Such evidences of the permanency of great natural phenomena arrest the attention of the physical inquirer with double force, when they present themselves in the ever-moving oceanic element. Although the limits of the fucus banks oscillate considerably, in correspondence with the variations of the strength and direction of the prevailing winds, yet we may still in the middle of the 19th century take the meridian of 41° W. from Paris (38° 38′ W. from Greenwich) as the principal axis of the “great bank.” In the vivid imagination of Columbus, the idea of the position of this bank was intimately connected with the great physical line of demarcation, which, according to him, divided the globe into two parts, with the changes of magnetic variation, and with climatic relations. Columbus, when uncertain respecting his longitude, (February 1493), directed himself by the appearance of the first floating streamers of weed (de la primera yerva) on the eastern margin 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 line, being made the celebrated “line of demarcation” between the Spanish and Portuguese rights of possession. (Compare my Examen Critique, tom. iii. p. 64–99, and Cosmos, English ed. vol. ii. p. 279–280.)