Drift Methods

Information as to the general direction of surface currents is obtained from the drift of floating objects such as logs, wreckage from vessels, and fishermen's implements. Thus, glass balls used by Japanese fishermen and wrecked Chinese junks are sometimes found on the west coast

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of the United States, and from these findings it is concluded that a current flows from west to east across the North Pacific Ocean. Another example is the recovery in July, 1884, from a drifting ice floe off southwestern Greenland, of equipment and documents from the ill-fated Jeanette, which on June 12, 1881, had been crushed in the ice to the east-northeast of the New Siberian Islands in lat. 77°17′N, long. 153°42′E. The recovery of these objects established the fact that water extends from Siberia to Greenland, and, since the relics were picked up on a piece of floating ice, the average speed of the drift across this sea could be ascertained. In most instances, conclusions as to currents by the finding of accidental drifting objects are incomplete, because the locality and the time at which the drift started are not known, nor is it known how long the object might have been lying on the beach before discovery. It is also difficult to determine to what extent such drifting bodies have “sailed” through the water, being carried forward by winds.

More than a century ago, in order to overcome such uncertainties, drift bottles were introduced. These are weighted down with sand so that they will be nearly immersed, offering only a very small surface for the wind to act on, and they are carefully sealed. They contain cards giving the number of the bottle, which establishes the locality and time of release, and requesting the finder to fill in information as to place and time of finding and to send this information to a central office.

In order further to insure reduction of the direct effect of the wind, drift bottles have sometimes been provided with a kind of drift anchor—for example, a cross-shaped piece of sheet iron suspended about 1 m below the bottle. In other instances, two bottles have been used, one of which has been weighted so much that it is carried by the other, the connecting wire between the bottles being about 1 m long. Still other experiments have been conducted with two bottles, one containing a weak acid which in a given length of time corrodes a metal stopper, thus permitting the sea water to fill the bottle. When this takes place, the bottles sink to the bottom, where they are held by a piece of sheet metal that acts as an anchor. This device has been used in the shallow waters of the North Sea, where bottom trawls are used extensively by fishermen, who recover many of the bottles.

The interpretation of results of drift-bottle experiments presents difficulties. In general, a bottle has not followed a straight course from the place of release to the place of finding, and conclusions as to the probable drift must be guided by knowledge of the temperature and salinity distribution in the surface layers. Fairly accurate estimates of the average speed of the drift can be made if the bottle is picked up from the water, or if a special drift bottle is brought up from the bottom. Bottles that are picked up on frequented beaches can also be used for estimating the speed of the drift. Tait's conclusions (1930) from the

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results of drift-bottle experiments in the eastern North Sea afford an example of ingenious interpretation. Numerous bottles thrown out simultaneously in about last. 57°N, long. 4°E were found on the coast of Jutland, the apparent time of drift in most cases being a multiple of twenty days. Tait assumed that off Jutland there was an eddy, as indicated by the distribution of salinity, and that the time for completing one circuit in the eddy was about twenty days. If most of the bottles had been drawn into this eddy and had escaped after having completed one, two, or more circuits, the equal time intervals at which the bottles were washed up on the beaches would be accounted for.

Drift bottles have been used successfully for obtaining information as to surface currents over relatively large ocean areas, such as the equatorial part of the Atlantic Ocean (Defant, 1929, p. 34) and the seas around Japan (Uda, 1935). They have supplied numerous details in more enclosed seas like the English Channel and the North Sea (Fulton, 1897; Carruthers, 1930; Tait, 1930), but have proved less successful off an open coast (Tibby, 1939).

The drift method can also be used for obtaining information as to currents in a shorter time interval. The currents derived from ships' records are determined by this method (p. 428) and give the average surface current in twenty-four hours or multiples of twenty-four hours. From an anchored vessel, say a lightship, the surface current can be determined either by a chip log (Bowditch, 1934, p. 11) or by drift buoys, below which, in general, there is a “current cross” acting as a sea anchor. This type of drift buoy was used on the Challenger. The latter methods give nearly instantaneous values of the surface currents at the place of observation.

Near land the methods can be elaborated in such a manner that the drift of a body can be determined in detail over long distances and long periods. A drifting buoy can be followed by a vessel whose positions can be accurately established by bearings on known landmarks, or the buoy can be provided with a mast and the direction to the buoy can be observed. Its distance from a fixed locality can then be measured by a range finder. Both methods have been used successfully. The latter can also be employed in the open ocean by anchoring one buoy, setting another buoy adrift, and determining the bearing of and the distance to the drifting buoy from a ship that remains as close as possible to the anchored buoy.

Still another drift method has been used with advantage in order to determine the ice drift in shallow waters out of sight of land. The method consists in letting a weight drop so rapidly to the bottom that it sticks in the bottom mud. The time and the length of wire rope payed out are recorded, and then more wire rope is payed out according to the drift of the ice floe to which the vessel is tied up. After a given length of

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time the wire rope is tightened and the total length payed out is recorded as the weight is pulled out of the bottom mud. The direction of stray of the wire is also recorded, and from these data the drift of the ice can be computed.