Escapes at outlets, in connection with modules, can be arranged by means of waste weirs or by means of Gregotti’s syphons (_sifoni autolivelatori_). The following is an abridged translation of part of a pamphlet by Gregotti:-- The figure represents one of the syphons installed in the “Centrali Milani.” A is the supply basin of the “Centrali,” which ends in the syphon B. The latter is constructed with mouthpiece of rectangular section _a_, which is submerged in the basin A. A weir divides the mouthpiece of the syphon from the descending branch, _c_, of the same, also rectangular in section. The weir crest is at level _dd_, from 2 to 7 cm. below the maximum level of water surface which it is desired not to exceed in the supply basin. [Illustration: FIG. 28.] The descending branch, _c_, has at its base a small tank _e_, which forms a water seal. The syphon is completed by a tube _f_, which is attached to the intake branch of the syphon and which ends at a level of 2 to 7 cm. above the previously mentioned surface _dd_. As soon as the water surface in the supply basin tends to rise above the plane _dd_, a filament of water, in falling over the weir _b_, pours down the descending branch _c_, and when the water has risen from 2 to 7 cm. above the crest of the weir, the thickness of the falling stream has become such that it is able, by lapping, with a wave-like course, the wall _gg_, to extract the air that has become enclosed in the syphon, and which cannot be replaced because the space in which the stream acts is closed at its base by the water in the tank _e_; and at the top also the aeration tube is closed by the rise in the water surface of the supply basin. From this point the syphon action quickly becomes fully established and begins to give its full discharge. The discharge that is given is equal to that of an orifice in a thin partition if certain limitations are allowed for between the fall used in the syphon and the height of the arch, that is, the distance from the crest of the weir to the inside roof of the syphon. The discharge is given by the formula Q = μA√(2_g h_). Q = discharge of syphon in cubic metres per sec. μ = a coefficient of reduction of discharge which varies between wide limits. A = the minimum cross-sectional area of the syphon in square metres. _g_ = value of acceleration due to gravity. _h_ = the fall, or the difference of level in metres between the water surfaces in the supply basin A and in the small tank _e_.