64 



DRAINING. 



retentive cl*ys, OT other poaching soils upon retentive 

 bottoms, forming cold or wet lands, which, as fur as 

 drainage is concerned, are very similar to the former. 

 These grounds are relieved by pro|H-r management of 

 the subsoil ; but, in treating of that division, we shall 

 take a view of the di-tcmperature of soils in general, as 

 well those that suffer from drought as from abundant 

 moisture. 



3d, Springy grounds, such as bogs, quagmires, and 

 quicksands, which arise from subjacent waters, and are 

 relieved by draining the strata or bottom. 



CII\P. I. Of Surface Draining. 



The principles upon which the drainage of low 

 grounds depend arc few ;uid simple, and may be readi- 

 ly comprehended, by attending to the ramification of 

 rivers and streams, which are the natural drains of every 

 country. 



The lowest attainable outfall must first be discovered, 

 and, if otherwise unobstructed, a main drain led up 

 from it into the space to be unwatered. From this, there 

 should be branches drawn into those which are not 

 commanded by the main drain alone. The junctions 

 with the main drain should be oblique, and pointing 

 down the stream, that the influx of side waters may not 

 tend to dam up those of the main. Into these branch- 

 es, the fence drains of the fields are conducted in inclo- 

 sed grounds, and the land is laid up in ridges and fur- 

 rows, terminating in the boundary fence drain. Where 

 the furrows are so situated as to run across a hollow, t Ill- 

 waters of which they are not deep enough to discharge, 

 the water furrow becomes necessary, and is drawn 

 across them in the hollow ground, so that a general 

 ventage may be given to every part of the surface by 

 an unobstructed fall, which is greatest at first, but may 

 be gradually diminished, as the quantities of water in- 

 crease, by uniting until the whole is discharged into the 

 sea. 



Method of In searching for the lowest outfall, the spiral level 

 finding the must often be resorted to. There are, however, natu- 

 bwe out- rjj marks, by which the fall of the ground, and conse- 

 quent direction of the flood waters, may be discovered 

 in the flat countries. On examining the ditches, which 

 become nearly dry in summer, it will be found that the 

 leaves of the various aquatics, as the sisymbrium, die wa- 

 ter cress, the veronica, or brook lime, &c. invariably 

 point down the fall, or in the direction to which the 

 water passes. This observation is well known to prac- 

 tical drainers. It should always be kept in view, as the 

 situation and direction of these plants will afford, in 

 many cases, a more ready means of discovering the di- 

 rection of the natural hollows than any other. During 

 flood also, it is of consequence to mark the progress of 

 the waters in their increase and diminution ; and it is 

 often advisable while they are stationary on the flats, 

 to sound them, by inserting many pegs in the ground, 

 so as to have their tops level with the water, by which 

 means the heights and hollows are at once perceived by 

 the different lengths of the pegs. This method is par- 

 ticularly convenient in laying out fields for irrigation 

 by flat flooding. 



w>KiiMi> With respect to the inclination or slope of drains, 

 r dope of there are certain limits which must not be exceeded; 

 the slope must lie such as will at least permit the water 

 to be carried off with sufficient rapidity to keep its 

 channel clear, but not so fast as to injure it by acting 

 on the sides or bottom. Mathematically sneaking, the 

 water should pass away with any the smallest inclina- 

 tion of channel, and even gradually accelerate in velocity. 



Uut we know that in fact thin tendency to accelerate Draining, 

 is speedil\ . by the friction ami other obstnic- 



ti.ms in the channel, anil these increase in proportion 

 to the sm.-illness of the quantity of wa- . at 



rivers are enabled to move witli less dccli\ily than is 

 required for smaller .streams. This lias not been suf- 

 ficiently attended to in the operations on the great 

 F.iiglisii fens; for, instead of mutiny the upper water* 

 into one capacious river, they have ln-en divided and 

 led away to different outfalls, greatly to the injury of 

 drainage. 



1. Large and deep rivers run sufficiently swift with 

 a fall of about 1 foot per mile, or . 1 in :>OOO 



2. Smaller rivers and brooks, with a fall of Si feet 

 per mile, or 1 in '^."lOO 



3. Small brooks hardly keep an open course under 

 4 feet 1 in 120O 



4. Ditches and covered drains require at least 8 feet 

 per mile 1 in GOO 



Furrows of ridges and filled drains, require much 

 more. 



The elevation of ridges measuring across, is, accord- 

 ing to the soil, sometimes as high as . 1 in 10 



Where the ground is level, and there is not in the 

 direction of the smaller drains a full equal to the above, 

 it may be given in the formation of the drain itself, by 

 cutting it deep at the outfall, which is preferable to 

 widening merely. 



The limit to the increase of the fall depends on the 

 cohesion of the stuff in which the drain is cut. Firm 

 rocky bottoms may be supposed to bear water passing 

 over them at any slope ; but, independent of the old 

 adage, that even the drop wears the stone, the boulders, 

 &c. which arc likely to be brought down by swift run- 

 ning streams, are, by their battering nnd rubbing, equal 

 to tile destruction of the hardest rocks; hence the beds of 

 mountain torrents are cut to great depths even in strata, 

 which appear able to withstand the effects of water for 

 ever. The stream will sweep away the substances that 

 form its channel until it leaves only those which have a 

 gravity in proportion to their weight snd figure, just 

 sufficient to counterbalance the impulse which it com- 

 municates. Now, the figure of the (K'bblcs l>eing sup- 

 posed the same, the weight of larger masses is greater 

 in proportion than the surface on which the water acts. 

 Thus, a pebble of twice the diameter, has four times 

 the surface, but eight times the weight. A table might 

 therefore be formed, which would exhibit the nature of 

 the bottoms of .streams of any given velocity. Such a 

 table tile reader will find in the article HIIIIX-.KS in this 

 work, being there intended to enable the builder to 

 discover the velocity of a river by inspection of the bot- 

 tom. But for the purposes of the drainer, it is to be 

 wished that a similar connection could be shewn be- 

 tweeen the slope of the channel and the nature of the 

 bottom which will admit of it. Now, from what has 

 already been said, it will ap|>cur that this must mainly 

 depend on the quantity of water which passes in the 

 channel, since we perceive that there is required for 

 good drainage a fall no less than eight times us great 

 in the small drain than would he requisite for the large 

 and deep river. The following Table may, neverthe- 

 less, be of some service; it is drawn from observation 

 on somewhat larger quantities than usually require to 

 be artificially conducted. The windings of the channel 

 and depth of water will, however, make great varia- 

 tions trom it. 



