In 1751 Henry Ellis, captain of the British slave-trading ship Earl of Halifax, was sailing across the North Atlantic Ocean when he made a surprising discovery. At 25° N, the latitude of modern-day Miami, he measured the temperature of the ocean waters. Taking measurements as deep as a mile, he found very cold water sitting below the warm tropical waters at the surface. Those water masses could have been cooled to such low temperatures only in an extremely cold environment, which indicated that they originated in polar regions.1
It wasn’t until decades later, in 1797, that Count Rumford (Benjamin Thompson) recorded, in his essay “Of the propagation of heat in fluids,” the important implication of those measurements: that they could be explained only through a North–South circulation system in the ocean. In the high latitudes of the North Atlantic, cold air during the winter months cools and densifies...
