Hope's apparatus consists of a vertical vessel full of water surrounded round the middle by a trough of cooling ice. Two thermometers, one above and one below the trough, measure the temperature of the water. It is designed to demonstrate that water reaches its maximum density at .
As the water in the central part of the vessel cools towards  (and thus becomes denser) it sinks to the bottom of the vessel, displacing the warmer water. The lower thermometer will then read a constant . Further cooling towards  will cause the now less dense water to rise to the top of the vessel, where the upper thermometer will then read a constant .