On Draining. 185 



swell with it. They usually retain, however, a greater quan- 

 tity than is necessary. This is particularly the case, when 

 they have a strong and impervious subsoil, through which 

 no water can penetrate. As this surplus quantity is inju- 

 rious to vegetation, it ought to be got rid of, either by surface 

 drains, or what are called hollow-drains. Sandy soils, on a 

 retentive bottom, also require draining, as the water cannot 

 descend lower, being lodged in the upper stratum ; but there 

 is seldom a necessity, for going deeper than a few inches into 

 the clay, on which the upper soil is incumbent. 



3. Land Springs. In many cases, soils are greatly inter- 

 mixed, and portions of sand and clay, or substances either 

 porous and retentive, are found in the same field. Draining, 

 in such cases, is attended with more difficulty, and requires 

 more skill, than where the superficial and internal strata 

 are thick, and regularly disposed. The means by which 

 this can be best effected, at a moderate expense, is to ascer- 

 tain the quality of the soil, by examining its produce. The 

 porous soils collect reservoirs of water, which augment in 

 times of rain, to the full level of the surrounding clay, from 

 which it bursts out, and forms a kind of temporary spring, 

 which renders the land over which it flows wet, and unpro- 

 ductive. It is then perhaps absorbed by anoilher porous 

 stratum, which produces similar effects. Formerly, this mis- 

 chief was endeavoured to be remedied by small drains, made 

 over the whole field, which seemed thus to be equally affect- 

 ed ; but by cutting a trench, from the nearest and lowest 

 part of the field to be drained, up to the highest and most 

 distant sand-bank, in such a direction, as to pass through 

 the intermediate sand-beds, the soil is radically cured. Be- 

 sides these main-drains, however, side^cuts are often neces- 

 sary. 



4. Springs from subjacent Water. A knowledge of the 

 causes, and the nature of springs, arising from subjacent 

 water, is so closely connected with the principles of drain- 

 ing, that it is necessary to explain it at more length. The 

 earth is known to be composed of various strata, which, 

 from differences in their nature and qualities, have ac- 

 quired the distinguishing names of porous and impervious. 

 Sand, gravel, calcareous earths, and various kinds of rock, 

 the parts of which are separated by frequent chinks and fis- 

 sures, are denominated porous soils. Clay of various sorts ; 

 certains kinds of gravel, with a mixture of argillaceous and 

 cementing particles in their composition ; and rocks of a 

 solid and compact nature, and without fissures, are the prin- 



