Grooves E E and a shutter F, as illustrated, for closing off the flow through the module, are provided, if required, but all modules are not fitted in this way, because many irrigation authorities consider it undesirable to provide the consumers with unrestricted facilities for closing off their supplies without previously giving notice of such an action. [Illustration: FIG. 30.--The Completed Module (Open Type for Low Heads).] An escape notch H is provided in the position indicated when desired. It may, however, be found difficult to determine beforehand the upstream water-level at which it is necessary to allow this escape of surplus supply, so that it is generally more satisfactory to cut the escape notch after the modules have been installed and actual experience has indicated a suitable level for the notch crest. In the standard type of module for irrigation purposes the top of the module chamber is completely open, as shown, and this is the type generally recommended, as it is found that consumers have greater confidence in an apparatus which hides nothing from them. To meet the needs of special cases, however, a second type is also made in which the chamber is completely closed and considerably reduced in height, being thus specially suitable for sites where space is confined. Pipes I, of diameter suitable for all sizes of modules, are also supplied. These may either be welded steel or cast iron, as desired. An 18-feet length of pipe is usually found sufficient to bring the supply through the canal bank to the module. All modules supplied are treated with anti-corrosive paint, which ensures the protection of the metal. APPENDIX K. KENNEDY’S GAUGE OUTLET.[66] (See p. 168.) [66] See _Punjab Irrigation Branch Paper No. 12_, “Results of Tests of Kennedy’s Gauge Outlet.” FIG. 31 shows a bell-mouthed orifice discharging into an air-space. The jet springs across the air-space and traverses a gradually diverging tube. Let _a_, A be the sectional areas of the stream at the air-space and the downstream end of the tube respectively, and let V, _v_, and P_ₐ_, P₁ be the corresponding velocities and pressures. Let resistances be neglected. Since the pressure in the air-space is P_ₐ_, V = √(2_g h_₀) or the discharge through the tube depends only on _h_₀ and not on _h_₁. [Illustration: FIG. 31.] By Bernouilli’s theorem, V² P_ₐ_ _v_² P₁ ---- + ---- = ----- + -- 2_g_ W 2_g_ W or P₁ - P_ₐ_ V² - _v_² --------- = ---------. W 2_g_ This quantity (since _v_ is small) is not much less than _h_₀ or V²/2_g_. In other words, the water levels of two cisterns with an air-space between them differ only a little, or _h_₁ is small.