Drifters, although modern, use a method of measuring current that is at least 200 years old. Some drifters are very simple, like the sealed bottle. Other more elaborate drifters send radio signals to instrument buoys, ships or satellites. Scientists use these signals to track the speed and direction of the currents that move the drifters.
The simple drift bottle contains a card for the finder to fill in with date and place of discovery and then return to sender. This method, while effective, has been discontinued by scientific organizations because of environmental concerns about the ones that aren't found.