MANUAL OF AGRICULTURE. 267 



absorb more of their heat than do the whitish, light-coloured sorts ; 

 but frora their property of radiation, they part with heat more 

 quickly than do the latter. Soils, exposed to the sun's hottest 

 rays at right angles, absorb more heat than if otherwise situated. 

 Clay soils, and such as have an excess of humus, absorb more 

 moisture and keep it longer than do sandy soils, and such as con- 

 tain a less proportion of organic matter. 



Drainage. — To the end that cultivated plants may flourish, it 

 is absolutely necessary that the soil be relieved from the pre- 

 sence of stagnant water ; for the presence of such wftter, be- 

 sides continually lowering the temperature of the soil, by the 

 constant demands it makes upon the soil's heat in the act of 

 evaporation, also precludes the possibility of air circulation 

 through the soil's component particles. The first step, accord- 

 ingly, in good husbandry, when the soil is not naturally dryish, 

 is its artificial drainage ; which process having been effected, and 

 the rain water now percolating through the soil into the drains, 

 and only so much of it as is retained in the soil by capillary 

 attraction becoming evaporable, the soil being thereby deprived 

 of less heat, has its temperature raised considerably, promotes the 

 earlier germination of seeds, and brings all the crops grown upon 

 it to a speedier maturity. Land is also thus rendered capable of 

 being worked at times and seasons in which it was formerly 

 unworkable, and always with a less expenditure of labour. The 

 common wavs of constructinfj the underground ducts of drainage 

 works in agriculture are sufficiently familiar. Clay pipes of 

 varying shape and bore, and laid continuously in parallel rows, 

 are usually employed, though in certain districts recourse is had 

 to flat stones, so arranged as to form a channel with triangular- 

 shaped bore. The surplus water then percolates the soil, and 

 meeting the drains, leaks or soaks through the joints imper- 

 fectly fitting of the clay or stone conduits, and by gravitation it 

 is carried through these to some clear outlet. 



Tlic Rotation of Crox>s. 



In like manner, as the strength of any mechanical chain is 

 regulated by its weakest link, so may the fertility of any given 

 soil be said to depend (other conditions being favourable) on that 

 one essential element or compound which is present in it in 

 the least quantity of all. To the recognition of this, and the 

 additional ascertained fact, that some plants require a greater 

 amount of i)articular elements or compounds than others do, 

 is (lu(j the custom, in agricultural practice, of observing a "rota- 

 tion" or varied succession of crops, whereby it is possible to 

 obtain permanently a maintenance of the i>roper balance of the 

 soil's constituents. The annexed table, composed by liarou 

 Liebig, in which the wliolc inorganic materials assimilated by 



