In general, the Gulf Stream as it issues into the sea through the Straits of Florida may be characterized as a swift current of highly saline blue water, whose upper stratum is composed of warm water. At its junction with ordinary sea water the edges may frequently be recognized in moderate weather by the ripples which occur as well as by the difference in color. Northward, in the cooler regions, the evaporation from its surface, when the temperature of the air is lower than that of the water, is apparent to the eye. In addition, in the lower latitudes, the stream carries with it a quantity of weed known as "gulf weed", which is familiar to all who have navigated its waters.

The axis of the Gulf Stream, or line of greatest velocity, lies from 10 to 20 miles eastward of the 100-fathom (183 m) curve.

Between Cuba and the Florida Reefs, off Habana, the axis of the stream is nearer the Cuban coast; but after making the bend between Salt Key Bank and Florida Reefs, the axis of the stream approaches the coast of Florida and lies from 4 to 11 miles outside the 100fathom (183 m) curve.

The mean surface velocity of the Gulf Stream is 3.5 knots at a point 111⁄2 miles east of Fowey Rocks and 2.2 knots at a point 60 miles south of Rebecca Shoal. These points are on the axis of the stream, or where the current is a maximum, the velocity of the stream decreasing gradually from these central points as the edges of the stream are approached. These velocities are affected considerably by prevailing winds.

The following tables give the mean surface velocity of the Gulf Stream in two cross sections in the Straits of Florida :

On issuing into the sea northward of Little Bahama Bank the Gulf Stream loses the relatively great velocity which characterized it within the straits. There is a gradual decrease to avelocity of about 2 knots off St. Augustine, Fla., in latitude 30° N. Here the Gulf Stream is joined by the Antilles Current, which flows northwesterly along the open ocean side of the West Indies before uniting with the Gulf Stream.

The Antilles Current, like the Gulf Stream, carries warm, highly saline water of clear indigo blue. The union of the two currents

gives rise to a broad current possessing about the same characteristics as the Gulf Stream within the Straits except that the velocity is much reduced. The combined current turns more and more easterly so that off the cost of Georgia the Gulf Stream bears northeast, maintaining this general direction past Cape Hatteras.

From within the straits the axis of the Gulf Stream runs approximately parallel with the 100-fathom (183 m) curve as far as Cape Hatteras, a distance of about 800 geographical miles, but at varying distances from the shore which sweeps northward in a sharper curve than does the 100-fathom (183 m) curve. Within the straits it is about 10 miles offshore; in the bight off the coast of Georgia the distance is about 100 miles; and at Cape Hatteras it is about 35 miles.

Earler systematic observations on the Gulf Stream dealt with the temperature of the water rather than its motion and the axis taken to be along the line of highest temperature obtained. Later the axis was taken to mark the line of greatest velocity. Ordinarily it is assumed that the two axes coincide but this is by no means certain. The thermometer, although it indicates the limits of the stream in a general way, is therefore only an approximate guide to the velocity of the current, nor is it to be assumed that the northerly set will be lost when the thermometer shows a region of cold sea water.

Within the straits the lateral boundaries of the Gulf Stream can be fixed with considerable precision. When the stream issues into the sea it is not difficult to define the western limits since the waters of the stream differ in color, temperature, salinity, and flow from the inshore coastal waters. On the east, due to the influx of the Antilles Current, the waters merge gradually with the waters of the Atlantic, and the limits of the stream cannot be fixed with precision. With regards to direction of flow, however, the limits may be fixed to include all waters flowing parallel to the axis but these limits vary with the seasons and with changing conditions of wind and weather.

Recent observations of the Coast and Geodetic Survey indicate that the inner limit of the Gulf Stream as far as Cape Hatteras may be approximately defined as the 50-fathom (91 m) curve.

Considering the outer edge as a line along which the current is still approximately parallel to the axis of the Gulf Stream, the width of stream northward of its outfall is as follows: Off Cape Canaveral about 70 miles; off the coast of Georgia after its union with the Antilles Current about 150 miles; off Cape Hatteras about 200 miles.

Off Cape Hatteras the wide-spreading current separates into numerous bands which is indicated by the juxtaposition of warm and cold bands of water of varying widths. This feature, though not so marked, is also noticed below Cape Hatteras.

It has been noted that the western or inner side of the Gulf Stream is separated from the coastal waters by a zone of rapidly falling temperature, to which the term "cold wall" has been applied. The abrupt change in the temperature of the waters separated by the cold wall is frequently very striking and is a definite indication of the edge of the stream. It is most clearly marked north of Cape Hatteras, but extends more or less well-defined, from the Straits to the Banks of Newfoundland. In the vicinity of the Banks of Newfoundland the cold wall represents the dividing line between

the warm current of the Gulf Stream and the cold waters of the Labrador Current which, according to observations, turns sharply, between parallels 42 and 43 and meridians 51 and 52, and flows easterly, parallel to the Gulf Stream.

Beyond the Banks of Newfoundland the identity of the Gulf Stream as such is lost but its general direction is preserved by reason of the prevalence of westerly winds, and its set toward Europe is continued until the continental shores are approached, when in the vicinity of the Azores, the current divides, one branch flowing to the northeastward and entering the Arctic regions and the other to the south and east in the direction of the African coast.

As previously mentioned daily and seasonal fluctuations as to location, velocity, and temperature have been demonstrated as occurring due to winds, and variations in barometric pressure and in temperature in the region through which it flows. A further cause of fluctuation is found in the variations of the currents which feed it or which, like the Labrador Current, come in conflict with it. Variations due to tidal currents are now considered as only minor in character.

A steamer bound from Cape Hatteras to Habana, or the Gulf ports, crosses the stream off Cape Hatteras. A fair allowance to make in crossing the stream is 112 knots in a northeasterly direction. for a distance of 40 miles from the 100-fathom (183 m) curve.

Crossing the Gulf Stream at Jupiter or Fowey Rocks, an average allowance of 211⁄2 knots in a northerly direction should be made for the set of the current.

Crossing the stream from Habana, a fair allowance for the average current between 100-fathom (183 m) curves is 1.1 knots in an east-northeasterly direction.

The approximate mean positions of the inner edge and axis (point where greatest velocity may be found) are indicated in the following table:

Approximate mean position of the Gulf Stream

At the western end of the Straits of Florida the limits of the Gulf Stream have not yet been well defined, and for this reason the location of the inner edge has been omitted for Habana, Cuba, and Key West, Fla., in the above table. Between Fowey Rocks and Jupiter Inlet the inner edge is deflected westward and lies very close to the shore line.

Along the Florida Reefs between Alligator Reef and Dry Tortugas the distance of the northerly edge of the Gulf Stream from the edge of the reefs gradually increases toward the westward. Off Alligator Reef it is quite close inshore, while off Rebecca Shoal and Dry Tortugas it is possibly 15 to 20 miles south of the 100-fathom (183 m) curve. Between the reefs and the northern edge of the Gulf Stream the currents are ordinarily tidal and are subject at all times to considerable modification by local winds and barometric conditions. This neutral zone varies in both length and breadth; it may extend along the reefs a greater or less distance than stated, and its width varies as the northern edge of the Gulf Stream approaches or recedes from the reefs.

The approximate position of the axis of the Gulf Stream for various regions is shown on the following Coast and Geodetic Survey charts: No. 1002, Straits of Florida; No. 1007, South Carolina to Cuba; No. 1112, Cape Canaveral to Key West; No. 1113, Alligator Reef to Habana. Chart No. 1001 shows also the position of the inner edge of the Gulf Stream from Chesapeake Bay to Straits of Florida.

WIND CURRENTS

There are given below the results of recent investigations on the currents caused by local winds. These investigations are based on observations made on a number of the lightships along the Atlantic coast from Nantucket Shoals Lightship to Brunswick Lightship. The results, therefore, apply more directly along the route between lightships but are applicable also to the coastal sailing routes farther offshore.

Direction of current due to wind.—It is evident that a wind continuing for some time will give rise to a current, the velocity of which increases with an increase in the velocity of the wind; and the mariner has taken it for granted that this current brought about by the wind sets in the same direction as the wind. But the results of careful observations show that this is not the case. Instead of setting with the wind, the current on the Atlantic coast of North America produced by local winds sets, on the average, about 20° to the right of the wind.

For example, a wind blowing from north will, on the North Atlantic coast, bring about a current that sets not south, but about 20° to the right of south, or 200°. Similarly a wind from south will produce a current setting 20° to the right of north, or 20°. It is to be noted that while the current due to the wind will, on the North Atlantic coast, set 20° to the right of the wind direction, the current which a vessel experiences at any time is the resultant of the combined actions of the tidal current, the wind current, and any other currents, such as the Gulf Stream or currents due to river discharge.

VARIATION OF THE COMPASS

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Velocity of current due to wind. The velocity of the current brought about by winds of different velocities is given in the table below. It will be seen that on the Atlantic coast of the United States the velocity of the wind current is about 12 percent of the velocity of the wind.

Table of current velocity due to wind-North Atlantic coast

An easily remembered working rule to get the velocity of the current due to wind along the Atlantic coast is to multiply the velocity of the wind (in miles per hour) by 11⁄2 and point off two places. This will give the velocity of the current in knots. For example, to determine the current due to a 40-mile wind we have 40×12=60, and pointing off two decimal places gives 0.60, or six-tenths of a knot. VARIATION OF THE COMPASS

The magnetic variation for 1936 and the annual increase or decrease at points mentioned are as follows:

The following data concerning the Intracoastal Waterway, from Boston, Massachusetts, to the Rio Grande, the route across Florida, the New York canal system, the Lakes-to-Gulf waterway, and the other river channels tributary to the Mississippi are included for ready reference.