The earlier navigators often found the Gulf-weed a serious impediment to their progress. Lærius mentions that for fifteen continuous days he passed through one unbroken meadow (Praderias de yerva, or sea-weed prairies, as Oviedo characteristically calls them), so that he could find no way through for oars. On certain occasions it has been found that the speed of vessels through the Sargasso Sea has been materially retarded; and it has been described as so thick that, to the eye, at a little distance it appears to be substantial enough to walk upon. That this is not the condition met with under all circumstances is proved by the fact that passing through this region in 1867, the writer made a seven days’ voyage through its central portion, during which the sea was at no time covered with the weed, so as to form a continuous meadow. It made its appearance usually in large patches, generally upon the surface, but sometimes apparently sunk to some distance below it. It varied considerably in appearance--was sometimes dark-colored, dense, and compact, and covered with berries; at others, pale and attenuated, with few berries. The masses, on some days were round and shapely, and usually scattered somewhat indiscriminately over the surface of the sea. Occasionally only a few small tufts appeared for many hours; and on one day the only sign of its presence was a long narrow streak, extending across the ocean as far as the eye could reach in the direction of the wind. The fact, indeed, is that the Sargasso Sea, dependent as it is upon a great physical phenomenon, changes its position according to the seasons, storms, and winds: its mean position remaining the same as it has been ascertained by observations during many years past. The Gulf Stream is the great power which maintains these marine pastures--a current whose impulse and origin, according to Humboldt, are to be sought to the south of the Cape of Good Hope--after a long circuit it pours itself from the Caribbean Sea and the Mexican Gulf through the Straits of the Bahamas, and following a course from south-southwest to north-northeast, continues to recede from the shores of the United States until, further deflected to the eastward by the banks of Newfoundland, it approaches the European coast. At the point where the Gulf Stream is deflected from the banks of Newfoundland toward the east, it sends off branches to the south near the Azores. This is the situation of the Sargasso Sea. Patches of the weed are always to be seen floating along the outer edge of the Gulf Stream. Now, if bits of cork, or chaff, or any floating substance, says Captain Maury, be put in a basin, and a circular motion be given to the water, all the light substances will be found crowding together near the centre of the pool, where there is the least motion. Just such a basin is the Atlantic Ocean to the Gulf Stream; and the Sargasso Sea is the centre of the whirl.