The mash passes from a tank as _A_ by a passage _a_ to an opening on one side of the central portion and into the outside channel _b_. The current of liquid passes along the outer channel until it is deflected by the central partition into the next interior channel _b_ and so on until it arrives at the center when it passes through the central partition into the other half of the chamber. Here it passes around back and forth and gradually outward to the outermost channel from which it passes off through an adjustable gate in outlet _c_. By adjusting this gate, and a gate or cock in inlet passage _a_, the passage and consequent depth of the liquid in the channels may be regulated. The vapor rising from the mash is carried over to a condenser through pipe _D_. In order to keep the mash from burning a chain _g_ is rotarily reciprocated along the channels by means of the bar _G_, the gear _E_ and the crank shaft _e_. Various modifications of this construction have been devised. The advantage of the still lies in submitting the mash in a thin current to the action of the heat, and the consequent rapid vaporization. [Illustration: FIG. 20.--Rotary Current Still.] Every distillation consists of two operations: The conversion of liquid into vapor, and the reconversion of the vapor into liquid. Hence perfect equilibrium should be established between the vaporizing heat and the condensing cold. The quantity of vapor must not be greater or less than can be condensed. If fire is too violent the vapors will pass out of the worm uncondensed. If the fire is too low the pressure of the vapor is not great enough to prevent the entrance of air, which obstructs distillation. As a means of indicating the proper regulation of the fire, the simple little device shown in Fig. 21 may be used. This consists of a tube of copper or glass having a ball _B_ eight inches in diameter. The upper end _E_ of the tube is attached to the condensing worm. The lower end of the tube is bent in U-shape; the length of the two bends from _b_ to outlet is four feet. The ball has a capacity slightly greater than the two legs of the bend. [Illustration: FIG. 21.--Indicator for Regulating the Distilling Fire.] Normally the liquid in the two legs will stand at a level. If, however, the fire is too brisk the vapor will enter the tube and drive out the liquor at _d_, and thus the level in the leg _C_ will be less than in the leg _D_. If, however, the fire is low, the pressure of vapor in the worm will decrease and the pressure of the outside air will force down the liquid in leg _D_ and up leg _C_ into the ball. A more perfected device but operating on the same principle is shown in Fig. 26.