The major surface currents of the oceans became known during the days of sailing ships when this knowledge was needed for successful ocean voyages. Information was accumulated by noting the course of drifting objects such as becalmed ships, drift-wood or pieces of wreckage. Oceanographers refined this technique by using specially designed drift bottles. These provided much of the original information about water movements around the British Isles. They are hardly ever used now but are described here for their historical interest and possible use in local inshore investigations.
A drift bottle has a long narrow neck and is usually ballasted to float with only the tip of the neck projecting above the surface, so that its course follows the movement of the water and is not much influenced by direct wind action on the bottle. The effect of the wind may be further reduced by attaching a small sea-anchor to the bottle. Inside the bottle is a postcard bearing an identification number and a request printed in several languages for the return of the card with details of time and place of finding, for which a small fee is paid.
Drift bottles can be used in several ways. They may simply be thrown overboard from an anchored vessel and their direction and speed of movement directly noted. This provides immediate information about the surface current in that locality at the time of observation. Usually they are set adrift in large numbers in the hope that some of them may eventually be found stranded, and their location reported. In this way, information regarding the general course of currents over wide areas and considerable periods has been obtained by a simple and inexpensive method. A variation is the use of ballasted drifters which hang below the surface or near the sea-bed. Strongly coloured dye can also be used although wind affects this more easily. In 1990 a lost cargo of about 40 000 pairs of Nike brand trainer shoes provided a free drift experiment in the North Pacific. Many of the shoes drifted ashore along the west coast of North America. An oceanographer, Curtis Ebbesmeyer from Seattle, realized their potential as 'drift bottles' and arranged to collate information on when and where the shoes landed up. Currently plastic yellow ducks, green frogs and blue turtles have reached the Bering Straits from a cargo of 29 000 bathtub toys lost in the Pacific in 1992!
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