PHYSICAL GEOGRAPHY— THE OCEAN. 



93 



This distinction between the portion which is 

 moved and that which is at rest, is most re- 

 markable where large quantities of sea-weed 

 carried along with the current permit us to esti- 

 mate its velocity. We occasionally observe 

 similar phenomena of limited currents in the 

 inferior strata of the atmosphere : after tem- 

 pests that have swept over dense forests, it 

 sometimes happens that the trees are only found 

 shattered and blown down in the course of nar- 

 row strips. 



The general motion of the sea between the 

 tropics from east to west, entitled the equato- 

 rial current, is regarded as a consequence of 

 the advancing times of the tides and of the 

 trade winds. It alters its direction in conse- 

 quence of the resistance of the east coasts of 

 the continents which it encounters in its prog- 

 ress. The new results which Daussy has ob- 

 tained from the motion of bottles thrown out 

 on purpose by navigators (10 French sea miles, 

 of 925 toises each, every 24 hours), argees to 

 within y\th of the velocity which I had ascer- 

 tained from a comparison of earlier data("9). 

 In the log-book of his third voyage (the first in 

 which he sought to make the tropics in the me- 

 ridian of the Canaries), Christopher Columbus 

 says : •' I hold it as certain that the waters of 

 the sea move with the heavens {las aguas van 

 con los cielos),^^ that is to say, from east to west, 

 like the apparent motion of the sun, moon, and 

 stars(3*o). 



The narrow currents, true oceanic rivers, 

 which take their way through the sea, run 

 warmer water in higher, colder water in lower 

 latitudes. To the first class belongs the cele- 

 brated Gulf-stream("^), which was known to 

 Anghiera(3*2), and particularly to Sir Humfrey 

 Gilbert in the 16th century. The commence- 

 ment and first impulse of this mighty current 

 is to be sought for southward from the Cape 

 of Good Hope, and it debouches from the Ca- 

 ribbean Sea and the Gulf of Mexico, through 

 the Straits of Bahama ; running from south- 

 south-west to north-north-east, getting farther 

 and farther from the shores of the United States 

 of America, it turns off eastward by the banks 

 of Newfoundland, crosses the Atlantic, and 

 frequently throws the seeds of tropical plants 

 (Mimosa scandens, Guilandina bonduc, Doli- 

 chos urens), upon the coasts of Ireland, the 

 Hebrides and Norway. The north-eastern 

 prolongation of the Gulf-stream contributes to 

 moderate the cold of the sea-water and also of 

 the climate about the north Cape of Scandina- 

 via. The warm Gulf-stream, after it has turn- 

 ed eastward from the banks of Newfoundland, 

 at no great distance from the Azores, sends 

 off a branch to the south, and it is here that 

 the Sargasso-sea, as it has been called, the 

 great bank of sea-weed, is met with, which 

 made so lively an impression on the imagina- 

 tion of Columbus, and which Oviedo called the 

 sea-weed meadow (Praderias de Yerva). A 

 host of small marine animals inhabit this ever- 

 verdant mass of Fucus natans, one of the most 

 widely diffused of the social plants of the 

 ocean, which is constantly drifted hither and 

 thither by the tepid winds that blow across its 

 surface. 



In contrast to the Gulf-stream, which belongs 

 almost exclusively to the northern hemisphere 



I of the Atlantic valley, and runs between Amer- ^ 

 ' ica, and Europe and. Africa, is the great cur- 

 rent of the Pacific Ocean, the inferior tempera- 

 ture of whose waters has an appreciable influ- 

 ence on the climate of the sea-boards along 

 which it sweeps, as I first observed in the au- 

 tumn of 1802(3*3). This current, in fact, brings 

 the dolder water of high southern latitudes to 

 the coast of Chili, runs along the shores of this 

 country and those of Peru, first from south to 

 north, and then (from the bay of Arica) from 

 south-south-east to north-north-west. In the 

 middle of the tropics at certain seasons of the 

 year the water of this cold ocean stream is not 

 higher than 15° 6 C. (60° F.), whilst the motion- 

 less water beyond its limits is as high as from 

 27° 5 to 28° 7 C. (81° 5 to 84° 6 F.). Where 

 the sea-board of South America, southward 

 from Payta, advances farthest to the west, the 

 stream turns suddenly in the same direction 

 from off the land, and takes a course from east 

 to west ; so that he who sails northward [by 

 crossing the stream] comes suddenly from a 

 colder to a warmer sea. 



It is not known to what depth the oceanic 

 currents, whether hot or cold, extend, how 

 near they run to the bottom. The deviation of 

 the South African current produced by the La- 

 gullas bank, where the water is full 70 or 80 

 fathoitis deep, appears to indicate a considera- 

 ble extension in depth. Sand-banks and shoals 

 outside the streams are mostly recogniza- 

 ble, as the excellent Benjamin Franklin dis- 

 covered, by the coldness of the water over 

 them. This depression of temperature appears 

 to me to be connected with the circumstance, 

 that with the communication of motion to the 

 neighbouring ocean, deep cold water is made 

 to rise over the edges of the banks and to mix 

 with the upper warmer water. My immortal 

 friend. Sir Humphrey Davy, on the other hand, 

 ascribed the phenomenon, from which the sea- 

 man can frequently draw practical inferences 

 conducive to his safety, to the descent of the 

 superficial strata of water cooled in the course 

 of the night : these remain nearer the surface, 

 because the shoal prevents them from sinking 

 to a greater depth. The thermometer was 

 turned by Franklin into a plumb-line ; fogs are 

 frequent upon banks and shoals : their colder 

 water causes precipitation of the vapour that 

 is dissolved in the sea air. I have observed 

 such fogs to the south of Jamaica, and also in 

 the Pacific, indicating the outline of shoals 

 sharply and quite distinctly from a distance. 

 They present themselves to the eye like air- 

 pictures, in which the fashion of the sub-mari- 

 time bottom is reflected. A still more remark- 

 able influence of these cold shallows is this, 

 that they produce an obvious effect upon the 

 superior strata of the atmosphere, almost in the 

 same way as low coral or sandy islands. Far 

 from all land, in the high seas, when the air is 

 elsewhere quite clear, clouds are frequently 

 seen hovering over the spots where shoals oc- 

 cur. In such cases their parings can be taken 

 by the compass, precisely as if they were lofty 

 mountains or isolated peaks. 



Without the variety of external forms that 

 characterize the surface of continents, the 

 ocean, when its interior is narrowly scanned, 



