592 PRACTICE OF AGRICULTURE. 



of induration, so as in some cases to become perfectly hard and rocky, but with frequent 

 breaks ot fissures passing through them. The loose, friable, marly strata are capable of 

 absorbing water, and of admitting it to filtrate and pass through them. 



4220. Thus the valleys "nd more lend grounds must constantly be liable to be overcharged 

 with moisturti and to become, in eon tequence, spouty, boggy, or of the nature of a morass, 

 accordingly as they may be circumstanced in respect to their situation, the nature of their 



.soils, or the nial. lids by which the water is obstructed and detained in or upon them. 



4221. Wliere lands have a sufficient degree of elevation to admit of any over-proportion 

 of moisture readily passing away, and where the soils of them are of such a uniform 

 aandy or gravelly and uninterrupted texture, as to allow water to percolate and pass 

 through them with facility, they can be little inconvenienced by water coming upon or 

 into them, as it must of necessity be quickly conveyed away into the adjacent rivers or 

 small runlets in their vicinity. 



4222. lint where grounds are in a great measure fat, and without such degrees of ele- 

 ction as may be sufficient to permit those over-proportions of moisture that may have 

 come upon them from the higher and more elevated grounds to pass readily away and be 

 carried off, and where the soils of the lands are composed or constituted of such materials 

 as are liable to admit and retain the excesses of moisture ; they must be exposed to much 

 injury and inconvenience from the retention and stagnation of such quantities of water. 

 Such lands consequently require artificial means to drain and render them capable of 

 affording good crops, whether of grain or grass. 



4223. Lands of valleys and other low places, as well as, in some cases, the level tracts 

 on the sides or borders of large rivers and of the sea, must also frequently be subject to 

 great injury and inconvenience from their imbibing and retaining the water that may be 

 thus forced to flow up into or upon them, either through the different conducting strata 

 from the hills and mountainous elevations in the neighbourhood, or the porous materials 

 of the soils. In these ways they may be rendered swampy, and have bogs or morasses 

 produced in them in proportion to the predominancy of the materials by which the water 

 is absorbed and dammed up, and the peculiarity of the situation of the lands in respect 

 to the means of conveying it away. 



4224. To perform properly the business of draining, attention should not only be 

 paid to the discrimination of the differences in regard to the situation of the lands, or 

 what is commonly denominated drainage level ; but also to the nature, distribution, and 

 depth of the materials that constitute the soils or more superficial parts of them, as upon 

 each of these some variety, in respect to the effects arising from water retained in them, 

 may depend. 



4225. The general origin of that wetness of land which it is the object of wider-draining 

 to remove, " will be found to be the existence of water in substrata of sand, gravel, open 

 rock, or other porous substances, which either lead to the surface, or, having no natural 

 outlet, become filled or saturated, while the pressure of more water coming from a 

 higher source, forces that which is in the lower part of the stratum upwards through the 

 superior strata to the surface ; thus occasioning either bursts and springs, or a general 

 oozing through the soil. The object in under-darning, therefore, is not to catch the surface- 

 water, but that which flows through their inferior strata ; and, for this purpose, it is 

 necessary to make a sufficient channel, either at the lower parts of the porous stratum, 

 or in such part of it as may most conveniently carry off the water, so as the pressure 

 referred to may be relieved, or the water intercepted before it reaches the surface. It 

 must always be kept in mind, then, that under-draining and surface-draining are oper- 

 ations essentially distinct ; and every care must be used in practice not to blend them in 

 the execution. If surface-water be allowed to get into covered drains, the sand and mud 

 which it will carry into these subterraneous channels will soon choke them up, and occa- 

 sion bursts, creating, as may be conceived, new swamps; while the expense of taking 

 up and relaying the under-drains will be very great, and the execution imperfect, the 

 sides being found never to stand a second time so well as when first formed." (Highland 

 Society s Trans, vol. vii. p. 218.) 



4226. Wetness of land, so far as it respects agriculture, and is an object of draining, 

 may generally depend on the two following causes: first, on the water which is formed 

 and collected on or in the hills or higher grounds, filtrating and sliding down among 

 some of the different beds of porous materials that lie immediately upon the impervious 

 strata, forming springs below and flowing over the surface, or stagnating underneath it ; 

 and, secondly, on rain or other water becoming stagnant on the surface, from the retentive 

 nature of the soil or surface materials, and the particular nature of the situation of the 

 ground. The particular wetness which shows itself in different situations, in the forms 

 of bogs, swamps, and morasses, for the most part proceeds from the first of these causes ; 

 but that superficial wetness which takes place in the stiff, tenacious, clayey soils, with 

 little inclination of surface, generally originates from the latter. 



4227. The most certain and expeditious method of draining, in such cases, is that of 



