256 ON THE DRAINING OF PLANTATIONS, 



lately, it was a prevailing idea among practical men that oblique 

 drains, whether covered or open, across the natural inclination or fall 

 of the ground, were the most effectual, in the belief that such drains 

 would in that way catch the water as it came down the slopes; and 

 many still consider this the proper course as regards open draining. 

 But as drains are made to receive infiltrated water from the soil as 

 well as surface water after heavy rains, the best system is to make 

 the drains run with the natural inclination of the ground. In this 

 way the drains have a good run or declivity, which is important to 

 promote an easy flow of the water collected, and they are also of 

 nearly equal depth on both sides, and so have the greatest drawing 

 or receiving powers in all directions, and from all quarters. On the 

 other hand, when drains are run obliquely or at right angles, on 

 moderately sloping ground, say at 24 or more feet apart, the bottom 

 level of the drains will be reached at a few feet from the under side, 

 while the water will have to descend several feet before arriving at 

 the level of the adjoining under drain, and such drains would therefore 

 in a great measure act from the upper side. Again, by the former 

 method, springs and wet patches are more sure to be tapped and 

 the water removed than by the oblique or cross system; the latter 

 in many cases misses them, even if of greater depth than drains cut 

 in a straight line down the slope, and so the ground is little bene- 

 fitted by the labour and money expended. But in the case of very 

 steep hills or mountain sides, where there is a great flow of water, 

 the perpendicular system may with advantage be deviated from, 

 especially as regards open drains, as the sides are liable to be under- 

 mined by the too rapid flow of the water. In such circumstances, 

 therefore, the drains should run in an oblique direction, not quite at 

 right angles, but at as acute an angle as practicable ; and it is advis- 

 able to shorten the length of the drains, and thereby lessen the 

 quantity and run of water, rather than have the drains at right angles. 



All drains should be made with as equal a gradient as possible, 

 so that they may run regularly, and not have a rapid and slow flow 

 alternately, causing them to be filled up and choked in the lower 

 places. 



The length of drains, open or covered, must be determined by the 

 fall of the ground and the quantity of water they are likely to 

 contain ; but in no case, except under special circumstances, should 

 covered drains run more than 300 yards, — and 200 yards will gene- 

 rally be sufficient for open drains ; while, if the flow of water is 

 strong, and the ground very steep, half that distance will be enough. 



