IRRIGATION AND DRAINING. 85 



elements of nutrition for plants, and especially contains con- 

 siderable proportions of carbonic acid and ammonia. If 

 these be permitted to percolate through the soil, the roots of 

 the plants, or in their absence, the elements of the soil itself 

 absorb and form permanent combinations with them. Air 

 also holds vegetable food, and it is necessary that this should 

 penetrate through every portion of the soil where the fibres 

 of the roots exist. Soils which are saturated with water, do 

 not admit of any air, unless the small proportion combined 

 with the water ; and from all such, this vital adjunct of 

 vegetation is excluded. The porosity of the land thus se- 

 cured, facilitates the admission and escape of heat, and this 

 last condition is of the utmost consequence in. promoting the 

 deposition of dews. 



The dense mass of saturated soil is impervious to air, and 

 remains cold and clammy. By draining it below the soil, 

 the warm rains penetrate the entire mass, and there diffuse 

 their genial temperature through the roots. Immediately 

 pressing after these, the warm air rushes in, and supplies its 

 portion of augmented heat to the land. Porous soils thus 

 readily imbibe heat, and they as readily part with it ; every 

 portion of their own surfaces radiating it, when the air in 

 contact with them is below their own temperature. This 

 condition is precisely what is adapted to secure the 

 deposit of the dews, so refreshing, and during a season of 

 drought, so indispensable to the progress of vegetation. 

 Dew can only be condensed on surfaces, which are below the 

 temperature of the surrounding air ; and rapid radiation of 

 the heat imbibed during the warmth of a summer's day, is 

 necessary to secure it in profusion for the demands of luxu- 

 riant vegetation, in the absence of frequent showers. 



An insensible deposit of moisture, precisely analogous to 

 -dew, is constantly going forward in deep, rich, porous soils. 

 Wherever the air penetrates them, at a higher temperature 

 than the soils themselves possess, it not only imparts to them 

 a portion of its exccess of heat, but with it also, so much of 

 its combined moisture, as its thus lessened capacity for re- 

 taining latent heat, compels it to relinquish. To the reflect- 

 ing mind, imbued with even the first principles of science, 

 these considerations will be justly deemed of the highest con- 

 sequence to the rapid and luxuriant growth, and full devel- 

 opment of vegetable life. 



Another essential benefit derivable from drained lands, 

 consists in the advantageous use which can be made of the 



