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National Geographic : 1912 Aug
Contents
771 THE GULF STREAM perature, has compelled steamship com panies to adopt lanes of travel and to make a detour around the region of dan ger. That the western-bound track was shaving dangerously near the limit of ice in the spring, when bergs are numerous, has been shown, and it is probable that the new lanes now used which lie fur ther south will be permanently adopted during the ice period. THE CAUSE OF OCEAN CURRENTS The theories as to the cause of ocean currents have been many. Columbus thought the stars, the air, and the waters of the sea all had the same motion around the earth from east to west and declared that the force of the equatorial current had washed away the land and thus formed the Windward Islands. Toward the end of the 17th century the belief seemed to be that all ocean cir culation was maintained by means of subterranean passage or abysses. A cur rent, upon meeting land, descended into the earth and ran through a tunnel to the other side of the obstruction. Strange to say, the writer was called upon in recent years to examine a paper written by a gentleman whose theory was somewhat similar to the above. He believed that all mountain ranges were simply the visible evidence of a tunnel conveying water from one ocean to another, the Rocky Mountain tunnel be ing the conduit by means of which water was transported from the Arctic to the Gulf of Mexico to form the Gulf Stream. Another theory was that the tropical sun evaporated so much water that the Afri can Coast current ran to fill up the hol low so formed. In recent times the course of currents has been laid to rivers and the Gulf Stream chiefly to the Mississippi. In act ual fact about 2,000 such rivers would be required. Some eminent men have attributed currents to the revolution of the earth, others to the differences in the density of the ocean at the equator and at the poles. Franklin's theory, which has many ad vocates at the present day, was that the winds produce the current by the air moving over the surface of the water, and he illustrated this theory by the fol lowing: "It is known that a large piece of water, o1 miles broad and generally only 3 feet deep, has by a strong wind had its water driven to one side and sus tained so as to become 6 feet deep, while the windward side was laid dry." As will be seen later, this is a well-taken ex ample of the force of the wind in caus ing the Gulf Stream, but it does not quite show the whole of the truth. None of these theories were based upon direct evidence by observations in the Gulf Stream, but all were inferences drawn from temperature of the water, from laboratory experiments, from the drift of vessels, or from reasoning based upon opinions of what ought to be. Much time and labor has been devoted toward attempting to define the limits of ocean currents and their velocities. Co lumbus on his first voyage, when nearing his final land fall, was trying to find the depth of the water one day, when he no ticed that the line inclined to the south west, from which he concluded that the surface was moving faster than the lower stratum which contained the weight on the end of the line. Franklin endeavored to use the ther mometer to define the limits of the polar and tropical waters, and hence the cur rent. This method is often correct, for without doubt tropical water is warmer than that coming from the poles, but it has been found that at times the warm tropical water may be blown by the wind over and onto the polar stream and then partake of its motion or, as in the Lab rador current, underrunning the warm water of the Gulf Stream. Temperature is not a sure indication of how the cur rent may be setting. Almost all governments at one time is sued instructions to their naval officers and requested the cooperation of the offi cers of their merchant marine to keep a record of the temperature of the surface water, and by the compilation of these data the supposed limits of most ocean currents were placed upon the charts. A methcd of determining the velocity of the currents has been in use since the introduction of comparatively accurate navigation. A vessel is moved at sea, by
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