In the usual modern type of canal fall in India the weir has no raised crest, and the water is held up by lateral contraction of the waterway just above the fall. The opening through which the water passes is trapezoidal (fig. 39), being wide at the water-level and narrow at the bed-level. In a small channel there is only one opening, but in a large canal there are several side by side, so that the water falls in several distinct streams. The curved lip shown in the plan is added to make the water spread out and cause less shock to the floor. The dimensions of the openings are calculated so that however the supply in the canal may vary, there is never any heading up or drawing down. The detailed method of calculation for finding C F and the ratio of A B to B C is given in _Hydraulics_, CHAP. IV. In cases where it is only necessary for the notch to be accurate when the depth of water ranges from B C to three-fourths B C, it will suffice to calculate as follows:--Let _b_ be the bed width of the canal, and let Q be the discharge and B the mean width of the stream when the depth of water is B C. Decide on the number of notches, and let W be the width of a notch calculated as if it were to be rectangular, _i.e._ by the ordinary weir formula. Increase the width to W´ = 1·05 W. Then make the notch trapezoidal, keeping the mean width W´, and making the bottom width _w_ (or C F), such that _w_/W´ = _b_/B. The top width of the notch is of course increased as much as the bottom width is reduced. [Illustration: FIG. 39.] A rapid has a long downstream slope, which is expensive to construct and difficult to keep in repair, especially as the canals can only be closed for short periods. Rapids exist in large numbers on the Bari Doab Canal in India, the face-work consisting in many cases of rounded undressed boulders--with the interstices filled up by spawls and concrete--which stand the wear well. Rapids have again been used on the more modern canals in places where boulders are obtainable, and where deep foundations would have given trouble in unwatering. The upstream face of a rapid is vertical, or has a steep slope. [Illustration: FIG. 40.]