A bottle, of one of various designs, that is released into the sea for use in studying currents. It contains a card, identifying the date and place of release, to be returned by the finder with the date and place of recovery. The bottle should be so designed and ballasted as to minimize direct wind effects. Since the path of a bottle can only be estimated between release point and recovery, and generally only a few percent are returned, this is an inefficient, although inexpensive, technique. Compare drift card.