I.—THE GULF STREAM.

As the results of the systematic investigation of the Gulf Stream upon a plan laid out by Dr. A. D. Bache, and executed, under his direction, by his most able assistants, have hardly yet been presented in a popular form, a sketch of the whole may not be out of place here. This investigation embraced not only surface-phenomena, but the whole internal structure and movement of this wonderful current. It is well known that the Gulf Stream has its origin in the equatorial current which, starting from the Gulf of Guinea, flows for a time in a westerly direction, till it approaches Cape St. Roque. This great projection of the eastern coast of South America interrupts its onward progress, and causes it to divide into two branches, one of which follows the coast of Brazil, in a southerly direction, while the other continues its course to the northwest, until it reaches the Caribbean Sea. After pouring into that basin, the great stream turns to the east to enter the Atlantic again off Cape Florida. The high temperature of the equatorial current is owing to its origin in the tropical zone, its westward course being determined by the rotation of the earth and by the trade-winds. On issuing from the Gulf of Mexico the stream is encased between the island of Cuba and the Bahamas on one side and the coast of Florida on the other. Here it meets the Atlantic in a latitude where the ocean-waters have no longer the high temperature of the tropics, whereas the stream itself has acquired an increased warmth on the shoals of the Gulf. This accounts for the great difference of temperature between the waters of the stream and those of the ocean to the east of it; while the still greater cold of the sea-water on its western side, between the Gulf Stream and the continental shore, is explained by the great Arctic current, pouring down from Baffin’s Bay, and skirting the shore of North America as far as the Coast of Florida, until it is lost in that latitude under the Gulf Stream. The object of Dr. Bache’s investigation was to trace the mutual relations of these two great currents of warm and cold water, flowing side by side in opposite directions, and to discover the conditions which regulate their movements and keep them within definite limits.

The investigation is even now by no means complete, though it has been going on for many years. It has, however, been ascertained that, while the ocean-bed deepens more or less rapidly as we recede from the shore, forming a trough in which the Gulf Stream flows, this trough is limited on its eastern side by a range of hills trending in the direction of the current, outside of which is another depression or valley. Indeed, the sea-bottom exhibits parallel ridges and depressions, running like the shore of the continent itself, in a northeasterly direction. The water presents differences of temperature, not only on the surface, but at various depths below. These inequalities have been determined by a succession of thermometric observations along several lines, crossing the Gulf Stream from the shore to the ocean water on its eastern side, at intervals of about a hundred miles. The observations have been made first at the surface, and then at successively greater depths, varying from ten to twenty, thirty, one hundred, two hundred, and even three and four hundred fathoms. This survey has shown that, while the Gulf Stream has a temperature higher than that of the waters on either side, it is also alternately warmer and colder within itself, being made up as it were of distinct streaks of water of different temperature. These alternations continue to as great a depth as the observations have been carried, and are found to extend even to the very bottom of the sea, where this has been reached. The most surprising part of this result is the abruptness of the change along the line where the two great currents touch each other. So sharp is this division that the boundary of the Arctic current is now technically designated as the “Cold wall” of the Gulf Stream. Of course as the latter flows northward and eastward it gradually widens, and its temperature is lowered; but even as far north as Sandy Hook the difference between its temperature at the surface and that of the surrounding waters is still marked.

Off Cape Florida the width of the Gulf Stream is not over forty miles; off Charleston it is one hundred and fifty miles; while at Sandy Hook it exceeds three hundred miles.

The inequality of the bottom may be appreciated by the soundings off Charleston, where, from the shore to a distance of two hundred miles, the following depth was successively measured: 10, 25, 100, 250, 300, 600, 350, 550, 450, 475, 450, and 400 fathoms.

The following table may give some idea of the temperature of the stream in connection with its depth:—

Off Sandy Hook, at successive distances from the coast, of
100,150,200,250,300,350,and400miles,
the temperature near the surface to a depth of thirty fathoms averages:
65°,66°,64°,81°,80°,and75°Fahr.;
at a depth of between forty and a hundred fathoms it averages:
50°,52°,50°,47°,72°,68°,and65°Fahr.;
at a depth below three hundred fathoms it averages:
37°,39°,40°,37°,55°,57°,and55°Fahr.

The rapid rise of the temperature after the fourth column of figures indicates the position of the Cold wall.

For further details see the United States Coast Survey Report for 1860, page 165, and the accompanying maps,—which should be copied into all our school atlases.