MECHANICAL PREPARATION OP SOILS. 37 



the locality, the soil and subsoil, the land-springs, and the average fall of rain 

 in the district. 



95. The object of drainage is to prevent water from stagnating in the soil, 

 by promoting its free percolation through it rather than by evaporation, which 

 has a tendency to reduce the temj^erature. The water to be withdrawn arises 

 from one of three sources, or from all three together ; namely, from land-springs, 

 or from rain, or from fountain-heads at a higher level. The engraving at the 

 head of this chapter represents one of these fountains, of a peculiar and some- 

 what mysterious character, — namely, a reciprocating spring. It is a sectional 

 view of the flowing well of the Peak, in Derbyshire, and, although differing 

 from most fountain-heads, it will serve to illustrate the subject. The commonly- 

 received explanation does not, however, explain all the phenomena ; it sup- 

 poses an interior cavity A, discharging its waters by a siphon-formed channel, 

 BCD. When the water is sufficiently high, it overflows the level C, running out 

 until the water is too low to be forced over the vertex of the arch of the siphon. 

 When forced into the channel, the waters descend through crevices in the rock, 

 or they force themselves a path to some porous strata, still descending, till they 

 reach some impervious rock or clay basin, where they again accumulate and 

 spread themselves until some new channel is found at a lower level. Supposing 

 this new basin to be impervious also, and the strata immediately over it to be 

 a porous gravelly soil, or a clay soil with patches of gravel, the water will 

 force its way upwards through those patches partly by the pressure of the 

 water at the higher level from which it is supplied, and partly by the force of 

 capillary attraction. This is generally the source of land-springs, which some- 

 times baffle the most skilful efforts at thorough drainage. 



96. Other land-springs, collected in tenacious subsoils having no outlet, 

 depend upon the fall of rain in the district. Water drawn from a higher 

 level can only occur where the land is overlooked by neighbouring heights. 

 In considering the amount of drainage requisite, it is to be borne in mind that 

 the process of evaporation is a powerlul agent in withdrawing water from the 

 soil, surrounding it with a cold damp atmosphere, inimical to vegetation ; 

 while the percolation through the soil assists in jDreserving its warmth. The 

 average fall of rain and evaporation in the ordinary soils round London may 

 be stated as follows : — 



Inches of Evaporation, 



Kaiu. in inches. 



From January to May 8-75 . . . . 4-45 



From May to August 7-0 .. ..6-80 



From August to October .. .. 5'50 .. .. 3*75 



In November 375 .. .. 0-GO 



In December .. .. .. .. 1'75 .. .. 0"17 



97. The first subject of inquiry on the eve of drainage is the nature of the 

 soil to be operated upon. Where its base is an aluminous clay, it will be 

 exceedingly tenacious of moisture, while a silicious dej^osit admits of very free 

 percolation ; from limestone rocks a chalky, friable, and moderately tenacious 



