The falls for drains arc governed by circumstances, such as when laid in flat and when in hilly districts. When not sufficient the drains will silt up, and when too great the inverts are worn by the scour of the grit, etc., carried along with the sewage In the latter case the tails have to be broken by steps. The minimum for drains should give a velocity of not less than 3 ft., and for sewers 2 ft. to 2 1/2 ft. per second. The maximum fall should give a velocity of about 4 /2 ft. per second. For a 3-in. drain the fall to give this latter velocity is about 1/30 for 4-in. drain, about 1/40in; for 6-in. drain, about 1/60; for 9-in. drain, about 1/30.