Dr Davy's Agricultural Discourse. 343 



oxygen which it contains, occasioning the decomposition of mineral 

 compounds in the soil and subsoil, and the setting free of inorganic 

 substances, those required for the purposes of vegetation, such as the 

 fixed alkalies, lime and magnesia, and certain acids, especially the 

 phosphoric, which plants in the act of growing are constantly ab- 

 stracting from the soil, by, if uncompensated, an exhausting process. 

 The penetrating water, impregnated with oxygen, is also beneficial, 

 in converting an injurious compound of iron, when present — the pro- 

 toxide — into the inert and harmless peroxide, and likewise, and in a 

 great degree, by favouring the decomposition of animal and vegetable 

 matter, and the production of carbonic acid and ammonia. For these 

 latter effects to be fully produced, the land should have the advan- 

 tage of thorough-draining. 



Water of various qualities is employed in irrigation, and, as might 

 be anticipated, with an effect varying with the quality, that depend- 

 ing on the substances suspended or dissolved in the water. The 

 purer the water, the less it will differ in its effect fi'om rain. The 

 more of decomposing animal and vegetable matter it contains, the 

 more the effect will be like that of rich manure frequently applied, 

 under the most favourable circumstances of season as to rain. The 

 more of earthy matter it holds in suspension, in a finely-divided state 

 — a state indeed necessary to suspension — the more its influence will 

 resemble that of a well-watered virgin soil. 



According to the kind of crop, water of irrigation, of one or the 

 other of these qualities, appears to be preferable. The rice-lands of 

 the mountainous parts of Ceylon yield, year after year, excellent pro- 

 duce, irrigated by water differing but little from rain water. The 

 vineyards of Zante and Cephalonia, the fruit of which is the currant- 

 grape, bear abundantly after a winter irrigation, the water used de- 

 scending from the hills discoloured by clay, an argillaceous, calcare- 

 ous marl, nnich resembling that deposited by the Nile, that vast irri- 

 gator and fertilizer of the ever-productive valley of Egypt. The 

 meadows in the neighboui-hood of Edinburgh, irrigated by the strongly 

 imjjregnated sewer-water of that city, are well known for the enor- 

 mous and repeated crops of grass they yield in the course of the year, 

 almost without intermission. 



The mode in which irrigation is performed is also various, de- 

 pending very much on the scale. If for garden and limited field 

 cultivation, in many countries the water used is raised from wells 

 or cisteiuis by the Persian wheel, or by the lever and bucket, and 

 distributed by little canals or gutters. If for extensive cultivation, 

 streams are conducted from lakes or rivers, and their water admitted 

 into prepared fields, and diffused over them. There are works for 

 this purpose in India, tanks and aqueducts of immense magnitude, 

 miles in circumference and length, which excite the wonder of the 

 passing traveller, and are, in the labour expended on them, little 

 inferior to the pyramids of I'^gypt themselves, it has been imagined, 



