"Well, that is a disputed point," replied the captain. "One authority says that the Gulf Stream `is caused by the motion of the sun in the ecliptic,' and I think there is a good deal of reason in this. Another philosopher puts it down to the influence of the anti-trade and passage winds blowing from the west to the east along the zone in which the stream travels; and I think much might be said about that argument, especially as the westerly current south of the tropic of Cancer is undoubtedly caused by the trade-wind. A third scientific gentleman ascribes the stream to the fact, that the earth being a globe, the water on the equator is higher than that of the tropics, and the lower stratum of fluid circles round constantly in its endeavour to reach into the bigger volume beyond its reach; but I can't say much for this theory myself, Tom." "But how do you know the Gulf Stream from the rest of the ocean?" I here asked. "As easily as you can distinguish a marlinespike from a capstan-bar," answered Captain Miles. "It is not only bluer than the surrounding water, through which it flows, as I've told you, like a river, but it is also several degrees warmer; for, when a ship is close to the stream and sailing in the same direction in which it is running, a bucket of water dipped from the sea on one side of the vessel will show an appreciable difference of temperature to that procured from the other. Besides, my boy, there's the Gulf-weed to tell you when you are within the limits of the current; however, you'll see lots of the weed by and by, no doubt, before we finish our voyage." "You said, captain," I observed, "that the great currents of the ocean are produced by the trade-winds?" "Undoubtedly," he replied. "Blowing with regular force on the surface of the sea, they cause it to move in the same direction in which they are travelling; and, this motion once acquired, the ocean stream keeps up its course far beyond where its original propelling power directly acted upon it. The `Great Equatorial Current' is produced by the south- east trade, the Gulf Stream, as I've just explained to you, by the western or passage winds; and the branch of the latter current that skirts the British Isles and Southern Europe, until it falls in again with the northern portion of the Equatorial Current, by the north-east trade-winds. Thus, the circle is completed, the water being ever in motion round the centre of the tropic of Cancer, just in the same way as the winds of this region are." "But what causes the trade-winds?" I next asked.