Two cylindrical copper vessels, equipped with iron feet and lids, are joined by two horizontal glass tubes.
By filling the containers with water and heating one of them, the motion of the water caused by the heating may be observed; the heated water becomes less dense and rises in the vessel, running through the upper horizontal tube; upon cooling and becoming more dense, it descends into the other container and returns to the first through the lower horizontal tube.
The principle shown by the device was exploited in the late 1700s to build the first heating systems with circulating hot water.

Device on display.