In paragraph XIII of the Pneumatica, Hero describes two vessels, E and F, placed on a base ABCD; from one, water flows out, while wine is poured into the other, and vice versa. Two curved siphons, GHK in E, LMN in F, communicate with the outside. Another siphon, XOPR, traverses the base putting the two vessels in communication and ends at their mouth, higher than the bent siphons GHK and LMN. Wine is poured into pitcher E, taking care not to exceed the level of the bent siphon H; otherwise, the wine would not flow out. But if enough water is poured into pitcher F to exceed the level of the bent siphon M, the water will begin to descend in tube RPOX and enter the other pitcher, causing the wine to flow out. Now liquid pours out of both vessels, from one wine, from the other water, until they are empty.