116 AGRICULTURE. 



cause, besides giving the necessary moisture, it fur- 

 nishes a considerable portion of alluvial matter. 



3d. Water charged with sand or gravel, or con- 

 taining iron or vitriol, or of a temperature very hot 

 or very cold, is unfavourable to vegetation, and 

 ought not to be employed, until, by standing in res- 

 ervoirs, it deposites these injurious matters in the 

 one case, and in the other acquires the temperature 

 of the atmosphere. 



4th. Clay and calcareous soils require less water- 

 ing than others. 



5th. Irrigation is of less importance in northern 

 than in southern latitudes ; and, 



6th. In cold climates, or in situations of much 

 elevation, irrigation is most usefully employed in 

 the spring and autumn; and in hot climates and 

 sandy soils in the summer. 



The principles peculiar to the two modes may 

 be collected from the following brief detail of the 

 labours necessary to each. In irrigating by sub- 

 mersion, the first and great labour is to make a dam 

 of ^euch strength as shall resist the volume of water 

 by which it may be pressed ; of such height as will 

 raise the water above the level of the ground you 

 wish to overflow ; and of such structure as will en- 

 able you to discharge the water it collects promptly 

 and entirely. The signal for doing this is the rising 

 of air-bubbles from the bottom of the pond, which 

 never takes place until a decomposition of the 

 plants below begins. In winter, this tendency to 

 decomposition is corrected by cold; and the sub- 

 mersion may, of course, be continued for weeks 

 and months, and the water permitted to freeze, not 

 only without injury, but with great benefit to the 

 plants, particularly if they have been closely pas- 

 tured in the fall. 



Filtration is a process requiring, in general, more 

 labour and science than the other; because, besides 

 a dam to raise a sufficient head of water, you must 



