In Gibb’s module a similar action is made to take place in a steel chamber, semicircular or spiral in plan, through which the water flows in a semicircular path instead of circulating round and round as in the teacup. The surface of the stream, however, assumes the same form as it does in a cup, because it flows under the same conditions. Across the chamber are fixed a number of vertical steel diaphragm plates which take the place of the teaspoon in the above analogy. The lower edges of these plates are of such a shape, and they are fixed at such a height from the bottom of the chamber, as to allow a stream of just the correct required discharge of water to flow under them without interference. But if, owing to an increase of head caused by a rise in the upstream water-level, the water tends to rise higher at the circumference of the chamber, then the water at the surface of the stream strikes against the diaphragm plates, and its centrifugal force being absorbed, it will fall in towards the centre just as happened in the teacup when the spoon was used in place of these plates. In this way the excess head that caused the additional rise of water at the circumference is used up by the fall back towards the centre. The full capacity of the semicircle or spiral for using up excess head or energy in this way is made available by the use of a sufficient number of diaphragm plates fixed at suitable intervals. When the range of head to be dealt with is not large, then a semicircular chamber is sufficient; but for large ranges of head the chamber is made of spiral form so as to lead the water round a complete revolution or more, as may be necessary. STRUCTURAL DETAILS. Fig. 29 shows the general form and structure of the type of module suitable for irrigation. Fig. 30 is from a photograph. The working chamber or shell A is constructed of mild steel plating securely riveted to a framework of angle steel, and the semicircular form of the shell with the rigid diaphragm plates B B riveted to the walls makes a very strong structure, and ensures durability. The “leading-in” bend C is of cast iron strongly bolted to the steel shell, and is so designed as to deliver the water into the module chamber in a completely established vortex condition. The socket D on this “leading-in” bend is made so as to allow of considerable latitude in the vertical alignment of the straight leading-in pipe, so that the water can be drawn from any desired depth in the parent channel, and the proportion of silt drawn off is thus brought under control. [Illustration: -- END ELEVATION. -- -- PLAN. -- -- FRONT ELEVATION. -- FIG. 29.--Details of Gibb’s Patent Module.]