Hope's apparatus

In physics, an apparatus used to demonstrate the temperature at which water has its maximum density. It is named after Thomas Charles Hope (1766–1844).

It consists of a vertical cylindrical vessel fitted with horizontal thermometers through its sides near the top and bottom, and surrounded at the centre by a ledge that holds a freezing mixture (ice and salt). When the cylinder is filled with water, this gradually cools, the denser water sinking to the bottom; eventually the upper thermometer records 0°C/32°F (the freezing point of water) and the lower one has a constant reading of 4°C/39°F (the temperature at which water is most dense).