288 



On the Drainage of Land. 



rities, besides draining the water chiefly from one point ; whereas 

 if placed in a slanting position^ they will intercept it in its descent 

 from every part of the land, and thus gradually collect a much 

 larger quantity, without carrying along with it anything to choke 

 their channel. Nor should the cross-drains be cut at right angles 

 to the main receivers, but made to join them in an oblique direc- 

 tion, tending downwards, so as to secure the course of the water 

 in its passage to the outlet. Their distance must of course be 

 regulated by the state of the land ; for if it be of a decidedly re- 

 tentive character, the drains will not act effectually if more than 

 20 feet apart ; and it has sometimes been found necessary to place 

 them at only 12 feet apart. If, however, it be of a porous species, 

 they will act at from 40 to 50 feet asunder; but from 18 to 30 

 feet may be considered a usual average/^ 



Notwithstanding what has been here said, it must be under- 

 stood that if the ground be nearly level, every advantage should 

 be taken of any declivity v/hich it may offer ; and if entirely flat, 

 the drains should be constructed with a gradual fall to the outlet 

 into which the water is to be discharged, which, whether ditch or 

 rivulet, should always have a deeper bottom than the drain itself. 

 To secure this regular descent, so that the v/ater may run from 

 one end to the other of the drain without its being in any way 

 impeded or suffered to stand dead, is a matter of serious impor- 

 tance. Although every one knows that water will flow at a very 

 slight inclination from its level, and it may be true that there is 

 more danger attending the greater than the lesser fall," yet the 

 exact rate at which the proportion of that inclination should be 

 measured in making drains on level ground — or the fall which 

 they should have in order to the attainment of their object — has 

 given rise to considerable variance of opinion among professional 

 men. Thus, in the " Penny Cyclopcedia," one foot is stated as 

 sufRcient fall for a drain of 300 feet in length, provided the drains 

 be not more than 20 feet apart. One writer assumes that a fall 

 of one inch in 20 feet is sufficient ; j while another insists that " no 

 drain should have a fall of less than one foot in 75;" J others, 



* It has been held as a rule to apportion the area of all drains to their 

 length, declivity, and distance from each other. Thus, supposing the length 

 to be 200 yards, and the distance from drain to drain 18 feet, the square 

 feet of surface receiving rain-water in twenty-four hours will give 1800 cubic 

 feet of rain-water; and taking the sectional area of the smallest, or '1^ 

 to 3 inch tiles, at 7" 5, and the water moving in this aperture at the rate of 

 one mile per hour, the number of cubic feet discharged by the drain in 

 twenty-four hours will be 6600, or nearly four times as much as is necessary 

 to carry off a surface fall of two inches of rain. See Prize Essays of the High- 

 land Soc, N.S., vol. vi. p. 94. 



t Carmichael, of Raploch Farm, on Tile-Draining. — Prize Essay of the 

 Highland Soc, N.S., vol. vi. p. 85. 



1 Thoughts on Draining.— Quart. Journ. of Agric, N.S., vol. iii. p. 88. 



