drift-cask

from The Century Dictionary.

• noun An empty cask closely sealed, usually furnished with a pole and flag, numbered, lettered, labeled, and tagged, set adrift in some part of the ocean, to be observed from time to time by passing vessels, as an aid in the study of ocean-currents. If it is found cast upon the shore, the inclosed tags, or labels, or cards are sent to the nearest hydrographic office. See drifter.