228 eOKBOtlBATION OF SOILS POSSKSSIKG NO CHOICE. 



In stony soils ... ... ... 1,220 per 1000 



„ stratified lock ... ... ... 1,840 „ „ 



„ massive rock ... ... ... 3,050 „ „ 



For a given velocity of current, the fall F may be determined 

 from the formula — 



F = h! 2 X P , 



(42-6375)^ w ' 



■where v = the given velocity of the water at the bottom of the 

 drain, p = the periphery of the section of the drain, and w = 

 the mean width of the section of water at its highest level. As 

 the drains may sometimes fill up to the very top, the value of w 

 should be the same as the mean width of the drain itself. 



The interval between two successive collecting drains will de- 

 pend on their depth, on the permeability of the soil, and on the 

 thickness of the layer of soil to be drained. The following proce- 

 dure for determining this interval with exactness in any given case 

 is recommended : — 



An experimental drain (Fig. 77, a,), of the proposed section, 



being cut, several holes (1, 2, 3, 4, ), of the same depth as the 



drain, and at short equal distances apart, should be successively 

 bored along a line at right angles to the direction of the drain. 

 The drain should then be blocked up until the water in it ceases to 

 rise any more. Both the drain and the holes will be found to have 

 filled up to the same level mm. Now open the drain and let it 

 work. The water in>the holes will gradually sink, and this the 

 lower, the nearer they are to the drain. In one of them it will 

 ultimately sink down to the depth to which it is proposed to drain 

 the soil. The distance of this hole from the drain will be equal to 

 half the distance that must be left between the drains. 



The depth of soil to be drained will of course depend on the 

 species to be introduce ; but, as a rule, it will not be much more 

 or much less than 3 feet. When the drains are close together, 

 the depth, and, therefore, the total amount of excavation is consi- 

 derably reduced by banking up the excavated soil between the 

 drains whereby the level of the ground gets considerably raised. 



SECTION II. 



Fixation of soils possessing no cohesion whatsoever. 



Such soils consist of silicious or calcareous sand with practically 

 no substances, such as clay, humus, &g., to bind the loose parti- 

 cles together into a cohering whole. The particles may be large 



