No. 15. 



Practical Jlgricnltiire. 



271 



them from the higher grounds, or sometimes they 

 are formed across the line of descent, in order to 

 intercept the water which runs from the higher 

 grounds to the lower- 

 In open drains, of whatever depth, the sides 

 should possess a declivity from the top to the bot- 

 tom, to prevent them from crumbling down and 

 being underminded by the current. Except in 

 the case of rock, this inclination should not be less 

 than 45 degrees ; and, when the earili is soft, and 

 the flow of water considerable, it should exceed 

 45 degrees. In all cases, the earth should be 

 spread from (he edge of the trench backwards, so 

 that the water from the land on each side may 

 have access to it. 



When drains of this class are covered, they are 

 generally made from 2^ to 3 feet deep, and filled 

 with stones or other loose materials to within a foot 

 of the surface. They are usually in this case car- 

 ried through hollow places, where the water of 

 tlie land stagnates, or tends to flow. 



The further end to be effected by draining is to 

 form channels for water which has already sunk 

 into the ground, and is either retained by it, or is 

 finding its way beneath the surface from a higher 

 to a lower level. It is the intercepting of water 

 below the surface that cnnsiitutes the most diffi- 

 cult part of draining, and which lequires the ap- 

 plication of principles which it is nrit necessary to 

 apply in the case of surface draining. 



When the soil rests upon a retentive subsoil, the 

 latter may present a surface of resistance to the 

 water; or the water may have sunk down into the 



subsoil, and be finding its way through the chan- 

 nels beneath. 



The substances through which water finds its 

 way with facility are the looser earths, sands, and 

 gravels, the crevices of rocks, and beds of loose 

 or decomposing stones ; the substances which re- 

 sist its progress are clays and the harder rocks. 



If we shall penetrate a little way into the looser 

 portion of the earth, we shall generally find a se- 

 ries of strata, consisting of gravel, sand, or clay, of 

 different degrees of density. These strata are 

 frequently horizontal, frequently they follow 

 nearly the inclination of the surface, and frequent- 

 ly they are broken and irregular. Sometimes the 

 stiatum is very thin, as a few inches in thickness, 

 and sometimes it is several feet thick ; and some- 

 times the traces of stratification disappear, and we 

 find only, to a great depth, a large mass of clay or 

 other homegeneous substances. 



When these substances are of a clayey nature, 

 water finds its way through them with difficulty ; 

 when they are of a looser texture, water perco- 

 lates through them freely. These last, according- 

 ly, form natural comluits or channels for the wa- 

 ter which is below the surface, when finding its 

 way from a higherto a lower level. 



When any bed or stratum of this kind, in which 

 water is percolating, crops out to the surface, the 

 water which it contains will flow out and form a 

 burst or spring, oozing over and saturating the 

 ground, as in the following figure, whice repre- 

 sents a section of the ground, from C to D. 



Fig. 44. 



When water is, in like manner, percolating through one of these pervious strata, and meets any 

 obstruction, as a rock or bed of clay at A, Fig. 44, it is stopped in its progress, and, by the pressure 

 of the water from a higher source, it is forced upwards, and thus saturates the superjacent soil, as 

 from D to E, forming springs, or a general oozing. 



In either of these cases, and they are the most frequeut that occur in practice, the object of the 

 drainer is to reach the water in its subterraneous channel before it shall arrive at the surface, and to 

 carry it away in a drain. 



-I» <, 





By cutting a drain at A, Fig. 44, the water of the stratum of sand CE, is cut off" before it reaches 

 the surface at C, where it forms the swamp CD. 



In like manner, in Fig. qf, by forming a drain at C or F, the water is cut off" in its channel AB, 

 and thus, in relieving the pressure from the higher source, by giving egress to the water through the 

 drain, the cause of the wetness trom E to D is removed. 



In looking :.t the sloping surface of any tract of ground, as a field, in which there is an oozing or 

 bursting out of water, we sha'l generally distinguish the line where the wetness begins to appear on 



