614 PROTECTION AGAINST SWAMPS. 



cold water. The clay used for these tiles should be fairly 

 pure, and contain no lime nor coarse grains of sand. The 

 larger tiles are used for main drains and the smaller ones for 

 collecting drains. 



ii. LAYING THE TILES. 



Trenches for the tiles are similar to those in use for open 

 drains, their dimensions depending on the nature of the soil ; 

 their sides, however, should be vertical ; the main drains have 

 a fall of 2 in 1,000, and the collecting drains a greater fall, 

 and should be from 30 to 80 feet apart according to the nature 

 of the soil. The shortest distance for collecting-drains is 

 chosen when there is much subsoil-water in stiff clay or in very 

 fine grained soil with great capillarity ; in fact, the looser the 

 soil, tlie further apart the drains should be. 



The depth of the collecting-trenches should be from 3 to 4J 

 feet, and at least 3J feet is necessary in cold climates, to 

 prevent damage to the tiles by frost ; the main drains should 

 be somewhat deeper. 



After the trenches have been dug, which for very wet soils 

 should be in fine weather and with as little delay as possible, 

 the tiles should be laid carefully, beginning at the upper end 

 of the drains, and placed end to end at the bottom of the 

 trenches ; or, if the soil be loose, on a layer of clay or on 

 flat roofing tiles, which will give the bed of the drain an even 

 gradient, to which much attention should be paid. Brown* 

 recommends placing a layer of stones under the tiles, in which 

 ordinary drainage- water may run, while its level will rise to 

 that of the pipes only when the soil happens to be exceptionally 

 wet. This is because the rootlets of trees are attracted 

 towards wet substances in the soil, and will therefore branch 

 freely among the stones and yet not block up the pipes, which 

 are usually dry. Brown also advocates placing a layer of clay 

 over the pipes, so as to keep the upper rootlets from getting 

 into them. 



The size of the aperture of the tiles depends on the degree 

 of wetness and capacity of the soil ; the more water there is 

 to be removed and the greater the danger of the tiles becoming 



* " The Forester." Brown and Nisbet. Blackwood & Co., 1894, Vol. I., 55], 



