John C. Swallow FRS (born in 1923-1994) was an English oceanographer who invented the Swallow float or sometimes referred to as a neutral buoyance

Neutral buoyancy

Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink...

 float, a scientific drifting bottle created based on the method used by shipwreck

Shipwreck

A shipwreck is what remains of a ship that has wrecked, either sunk or beached. Whatever the cause, a sunken ship or a wrecked ship is a physical example of the event: this explains why the two concepts are often overlapping in English....

ed sailor

Sailor

A sailor, mariner, or seaman is a person who navigates water-borne vessels or assists in their operation, maintenance, or service. The term can apply to professional mariners, military personnel, and recreational sailors as well as a plethora of other uses...

s who placed and sealed messages in bottles and hope that the said bottles will reach any inhabited shores so that people can help them.

Swallow's invention

Swallow's float was invented so that it would go underwater but keep afloat at a particular depth. The people on board the ship that follows the bottle should be able to know its location at all times. The Swallow float was also created with the ability to make sound signals into the water. Such sound signals would be heard and located by scientists aboard the ship, including the location of the bottle itself. Because of this apparatus, scientists were able to learn how water flows in the deep ocean. Many surface currents, the Gulf Stream

Gulf Stream

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean...

 for example, have countercurrents or currents

Subsurface currents

A subsurface current is an oceanic current that beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep thermohaline circulation in the Atlantic, and bottom gravity...

 that flow under the surface currents. Countercurrents flow by going to the opposite direction but within the same path flow of the surface current. Because of Swallow's float, scientists were also able learn to that there is no steady water circulation within the deep ocean; this is because, sometimes, the water keeps going round and round in a huge whirling action creating an eddy

Eddy current

Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...

which drifts along slowly.