The above case (two cisterns and air-space) is mentioned in _Hydraulics_, Chap. V. The principle is simply that the velocity head at the air-space is reconverted into pressure head by passing the stream through a gradually diverging tube. In the absence of such a tube the velocity head would be wasted by causing eddies in the downstream cistern. If the downstream cistern is a watercourse whose water-level is considerably lower than that of the upstream cistern or distributary, V is obviously unaffected. Also P₁ is obviously reduced. Therefore, by Bernouilli, _v_ is increased, or the stream does not fill the expanded tube and there are eddies in the tube. The water-level in the watercourse may even be lower than the end of the tube. The discharge is unaffected. In practice there are, of course, resistances, but this fact does not affect the general conclusions stated above. The minimum working head (difference between the two water-levels) which gives a constant discharge is greater than would be the case in the absence of resistances. This “minimum working head for modularity” has been found to be ·21 foot, ·42 foot, and ·61 foot, the corresponding values of the “depression,” _h_₀, being respectively 1 foot, 2 feet, and 3 feet. When the working head is less than the above, the discharge is less and it depends on the working head. The depression should, according to Kennedy, be about 1·75 feet, but it may be more. The chief difficulty in using the gauge outlet as a module is that the air vent can be stopped up. This converts the apparatus into a compound diverging tube (_Hydraulics_, Chap. III., Art. 17). The discharge is, of course, increased, and it becomes dependent at all times on the working head. Another difficulty is that any rise or fall in the water-level of the distributary (and such rises and falls may occur owing to silting or scour, however carefully the discharge may be regulated) alters the discharge somewhat, though not to the same degree as in an ordinary outlet with a working head of, say, ·5 foot. In short, Kennedy’s gauge outlet, or “semi-module” as it is sometimes called, can modify but not do away with the variations of the discharges of outlets. INDEX. Absorption, 16, 159. Alignment, principles of, 4. Alignment, centrality in, 5. Alteration in line, 59. Assiut Barrage, 11. Assouan Dam, 11. Banks, construction of, 138. -- protection of, 139, 175, 178. -- width and height of, 56. Banks and Roads, 53. Basin Irrigation, 11. Berms, 53. Bifurcations, 47. Bifurcation, head needed at, 45. Borrow pits, 55. Branches of Canals, 3. Breaches in Banks, 176. Bricks used for canal work in India, 88. Bridges, 8, 80, 87, 130. -- Skew, 29, 42. Bushing of banks, 178.