Next we turn to a feedback loop that again includes a human element, but with a difference. Here man does not have the controlling functions of sensing and corrective action; instead, he plays the role of the disturbance. The purpose of the device is to limit arbitrary human actions: it regulates the wine consumption of the participants in a drinking bout. The report on the drinking straw regulator of the Ch’i Tung natives in southern and southwestern China is given in the travel journal Ling Wai Tai Ta by Chou Ch’u-Fei (died 1178):

They drink wine through a bamboo tube two feet or more in length, and inside it has a “movable stopper” (kuan li). This is like a small fish made of silver. Guest and host share the same tube. If the fish-float closes the hole the wine will not come up. So if one sucks either too slowly or too rapidly, the holes will be [automatically] closed, and one cannot drink.

The device was designed to enforce a uniform rate of drinking, i.e., to maintain a constant rate of flow. The desired flow is determined by the weight of the float. If the upward draft is too strong, the hydrodynamic lift overcomes the weight of the float; the float will rise, stopping the flow of wine. A maximum flow rate of wine, however, will be obtained by a drinker who does not draw as hard as he can, but who instead optimizes the suction by intuitively finding the derivative dq/dp = 0. At this suction pressure a steady flow rate will establish itself. The drinking-straw regulator therefore again does not represent genuine feedback control, for it achieves equilibrium only when used by a man who, by a process of optimization, forms a second closed loop. Historically the drinking-straw regulator is unique. The Ch’i Tung natives were quite isolated from the influences of foreign cultures because of their geographical situation; the invention must be their own. In Europe, control devices of this type were patented, perhaps for the first time, by Achille Elie Joseph Soulas in 1841 (Brit. Pat. No. 8894).