"You are not the first person, Harry, who has taken that collection of sea-weed for land," he observed.  "That is the Sargasso Sea.  When the companions of Columbus sighted it, they thought that it marked the extreme limits of the navigable ocean.  We are at the southern edge of it.  Look at this chart; it extends in a triangular form between the groups of the Azores, Canaries, and Cape de Verds.  It is caused by the Gulf Stream, which, circling round the Atlantic, sends off towards the centre all the sea-weed and drift-wood collected in its course.  Throw some chips into that tub; now, set the water in motion with your hand. The current you have created sends off all the chips into the centre of the tub.  You need never forget how this Sargasso Sea becomes covered with weed.  But you will wish to know something about this wonderful Gulf Stream, which not only produces the effect I have described, but exerts a very powerful influence, on the climate of many countries, and on the navigation of the Atlantic, besides causing many other important results.  It is, indeed, one of the most wonderful of all the phenomena of the ocean.  Consider it as a mighty river of warm water flowing for three thousand miles with scarcely diminished volume, never dying, never overflowing, over a bottom and between banks of cold water.  So little affinity have its waters with the common water of the ocean, and so different is their colour, that a distinct line can often be traced where they pass along.  See where it takes its rise in the Gulf of Mexico, whence it is called the Gulf Stream.  Now, mark its course, and note its effects.  Remember, that not only is it warm itself, but it warms the air which passes over it.  It likewise contains much more salt than the common sea-water.  The salt gives it its peculiar deep indigo-like colour.  It runs at the rate of between three and five miles an hour.  It is roof-shaped--that is, higher in the centre than on either side.  This is proved by placing a boat on either side of the centre, when it drifts off towards the edge nearest to which it is cast loose.  Another peculiarity exists in connection with it.  Water radiates heat far more slowly than does the earth.  If, therefore, the Gulf Stream swept along the ground, it would speedily lose its heat.  To prevent this, it is made to pass over a cushion of cold water, into which its heat does not readily pass.  When, however, its waters wash any shores, they impart some of their heat to them, increasing the warmth of the climate, adding fertility to the soil, and making it a more agreeable abode for man.  Now, look at the chart, and observe where the mighty current leaves its reputed source in the Gulf of Mexico. Mark it sweeping round the coast of Florida, and glancing off to the eastward near Cape Hatteras, in the United States, allowing a belt of cold water to wash the shores of that country during the winter months of the year.  Watch it passing near the coast of Nova Scotia, and in the summer, not far from that of Newfoundland, where it has undoubtedly caused the formation of the well-known fishing-banks.  This is the way they have been produced.  When the summer sun releases the innumerable mighty icebergs which have been formed on the shores of the polar regions, they float away to the south, carried by a current which sets towards Newfoundland.  They bear away with them vast quantities of rock, and stones, and sand.  Meeting the hot water of the Gulf Stream, they quickly melt and deposit their burdens at the bottom, always about the same spot which you see marked as the Grand Bank.  Now the stream, taking an easterly course, reaches the 40th degree of north latitude, when it begins to spread itself over the colder water of the ocean, washing the shores of Ireland; some going up towards Spitzbergen, surrounding the Shetland Isles, and other isles in the north; more rushing up the British Channel; and another quantity flowing into the Bay of Biscay, and away again towards the south--adding warmth to the whole of the indented shores of Europe, and at the same time supplying the deficiency of salt to the waters flowing out of the Baltic and the Polar basin."