Field Drainage. 45 



The object being to get rid of the water quickly, the less 

 run it has through the small pipes the more rapid will be the 

 discharge, the friction in the mains being much less than in 

 the smaller drains. And so, in a very large field, it is never 

 desirable to lay the smaller drains of a greater length than 

 200 yards. Some engineers allow 300 yards as a maximum 

 length, and instances have come under the author's obsen^a- 

 tion where 2-inch pipes laid in a clay soil in lengths of 20 

 chains have been in effective working order for the past ten 

 years, and will possibly remain so as long as the pipes last ; 

 but under ordinary circumstances, 10 chains may be taken as 

 the maximum safe working length. 



In flat districts it is seldom that much fall can be given to 

 the pipes, but a very slight inclination will cause the water to 

 travel with sufficient rapidity to the outfalL It is better that 

 the drains should be laid perfectly level than that a fall 

 should be acquired by laying the pipes shallow at one end 

 and deep at the other, the advantage gained by a fall thus 

 acquired being neutralized by the varying effect the differ- 

 ence of depth must have on the uniform diying of the ground. 

 The distance the drains are apart is determined with refer- 

 ence to the depth ; therefore if the drain be laid shallow at 

 the upper and deeper at the lower end, the distances must 

 either be too great at one end or too little at the other. 



Fall is not absolutely necessary to the safe working of 

 drains. By the action of gravity, water is attracted towards 

 the earth's centre, and travels towards that point until its 

 progress is arrested by some impediment. Water varies 

 from more solid substances in that all its particles are free to 

 act, and they have so little cohesion, that every particle is free 

 to obey the influence of gravity, and seeks the lowest place 

 it can find ; a natural fall is thus caused on the surface of 

 the water in the drains. A fall in the drain itself only assist^ 

 this action, because all falling bodies acquire a velocity in 

 proportion to the height from which they fall ; and so the 



