187 1. J The Theory of Irrigation. 41 



from land-drainage, both as they affect the temperature and 

 humidity of the atmosphere and soil ; which, in their turn 

 are, with a good show of reason, supposed to have a con- 

 siderable effect upon the distribution of rainfall, though not, 

 perhaps, upon the actual amount of it. It is impossible to 

 restore the harmony of nature thus once disturbed, without 

 allowing the lands, cleared and improved, to revert to their 

 original state ; but as this would be detrimental rather than 

 conducive to man's interests, it is more desirable that the 

 balance should be restored in other ways, and by other 

 means, which, whilst counteracting the evil effects above 

 referred to, admit of the retention of the land in its improved 

 state of productiveness. Thus, by the artificial production 

 of moisture in the soil, by means of irrigation, the equilibrium 

 may be restored ; whilst the subsoil drainage which has in 

 many cases rendered a resort to irrigation necessary, is in 

 itself essential to the proper development of cultivation by 

 irrigation ; otherwise the land, especially in heavy soils, is 

 liable to become waterlogged, to the injury alike of the crops 

 and the health of the neighbourhood. This latter is clearly 

 proved in the case of rice crops, which are so notoriously 

 injurious to health that no European can with safety sleep 

 in their vicinity. " Not only does the population decrease 

 where rice is grown," says Escourron Milliago, " but even 

 the flocks are attacked by typhus." This is happily not the 

 case where simple irrigation is adopted for the growth of 

 grass, cereals, vegetables, and other crops required in 

 European countries generally, where proper attention is paid 

 to subsoil drainage. The reason why land will not produce 

 good crops in the absence of a sufficient amount of water, 

 even though it be highly manured and otherwise well 

 cultivated, is that moisture is essentially necessary for the 

 admixture with the soil of those invigorating properties 

 existing in manures, which, in the absence of that agency, 

 would, though mechanically mixed with the earth, remain 

 chemically separate and distinct from it, and, therefore, not 

 in such a state as to be in any way beneficial for the develop- 

 ment of growth in herbage or plants. With the assistance 

 of water, however, the salts contained in manure are set 

 free and eagerly unite with the soil, by which they may be 

 said to be digested and prepared to become fit food for the 

 nourishment of vegetation ; but, even when so taken up, 

 these salts are, during seasons of drought, held from vegeta- 

 tion with an iron grasp by the soil, from which moisture 

 alone can again loosen them. Thus we see that, whilst 

 moisture is required in order to cause a chemical combination 



VOL. VIII. (O.S.)— VOL. I. (N.S.) G 



