46 The Drainage of Fens and Low Lands. 



greater the fall the greater the velocity, and the greater the 

 velocity the greater the power to overcome obstacles, and the 

 more certa^Jn and rapid the discharge. Thus, while fall in the 

 bottom 9! the drain is a great advantage, and even a necessity, 

 in dr^ns which convey water having matters In suspension, as 

 in/town sewers, in enabling the water to keep the drain free 

 irom deposit, it is not absolutely necessary to the discharge 

 of clear water, or for land drains laid at a proper depth ; 

 and many miles of drains have been laid that are now doing 

 their work well which have not an inch of fall 



Where pipes are laid level, or where only a very slight fall 

 can be obtained, the main should, wherever possible, be laid 

 lower than the small drains, and the end pipes should always 

 tip, or be laid at a greater inclination than the others, in 

 order to assist in drawing off the water. 



The connections of the drains should never be made at a 

 right angle, but the smaller pipes ought always to be made 

 to enter the mains with a curve, or at a veiy obtuse angle. 

 When one current of water impinges on another at a right 

 angle, it causes a stoppage in both, and hinders the flow : an 

 eddy is thus created, and any heavy matter held m suspen- 

 sion is precipitated, having a tendency to choke the pipes ; 

 whereas, if the smaller stream has the same direction given 

 to it as the larger, by the pipes being made to join the others 

 with a curve, the united currents flow on together without 

 interruption. Experiments made in sewer work resulted in 

 ascertaining the fact that, when equal quantities of water 

 were running direct, at the rate of 90 seconds ; with a turn at 

 right angles, the discharge was only effected in 140 seconds ; 

 whilst with a turn or junction, with a gentle curve, the dis- 

 charge was effected in 100 seconds. 



Irregularities in the cutting of the trenches and in the form of 

 the pipes are far more injurious to efficient drainage than 

 want of fall. It is of the first importance that there should 

 be no hills and holes in the bottom of the trenches, but that a 



