1 87 1.] The Theory of Irrigation. 39 



part of their water, sometimes suddenly swell, and that, 

 occasionally, to a degree which causes great disasters. 

 Besides, after violent storms, springs which had become 

 almost exhausted have been observed to burst out with 

 impetuosity, and soon after to dry up again." Arguing from 

 the basis of facts already established, he draws the conclu- 

 sion that forests have a special value — " that of regulating, 

 of economising in a certain sort, the drainage of the rain- 

 water." 



To sum up the results consequent upon the clearance of 

 forests, as already set forth, it may briefly be stated that 

 any undue extent of interference with the economy of nature 

 in this respect cannot but be followed by the drying of the 

 vegetable mould on the surface of the ground affected by the 

 clearance ; and it soon becomes removed by the alternate 

 action of wind and rain, leaving behind a sterile soil, 

 possessing none of the properties necessary for cultivation ; 

 but not, fortunately, beyond the power of man to restore, 

 in course of time, to its former powers of reproduction. The 

 means for effecting this are the same which, if adopted earlier, 

 would have prevented its falling into a state of sterility, viz., 

 the artificial application of water to the soil, so as to 

 counteract, in some measure, the consequences necessarily 

 arising from an interference with the proper proportion pre- 

 scribed by nature of forest to open land. 



Having now considered the effects caused by the destruc- 

 tion of forests, we have, in the next place, to trace, in a 

 similar manner, the probable evil consequences of land 

 drainage. 



Surface-drainage is a necessity in all newly-reclaimed 

 lands, and probably dates its origin from the commencement 

 of agriculture ; but the construction of subterranean channels 

 for the removal of infiltrated water, marks ages and coun- 

 tries distinguished by a great advance in agricultural theory 

 and practice, a large accumulation of pecuniary capital, and 

 a density of population which creates a ready demand and a 

 high price for all products of rural industry. Under-drainage 

 being most advantageous in damp and cool climates, where 

 evaporation is slow, and upon soils where the natural incli- 

 nation of surface does not promote a very rapid flow of the 

 surface waters, it is not surprising to find that this practice 

 has been carried further, and a more abundant pecuniary 

 return obtained from it, in England than in any other country. 



By removing water from the surface of the soil, however, 

 the amount of evaporation is necessarily lessened, and the 

 refrigeration which accompanies all evaporation is diminished 



