WORK OF FORMATION. 43 



rain-drops are highly oxygenated. Plants will not grow in ground 

 saturated with water from which the oxygen has been more or less 

 taken up, and drainage is therefore necessary in promotion of vigorous 

 vegetable growth. Such drainage 'may be consequent on natural 

 formation of the ground, or be secured by the removal of obstacles, 

 or by artificial means. But the cold stagnant water that has, as it 

 were, served its purpose must be removed, or conducted to where 

 surrounding conditions admit of its return by evaporation to discharge 

 its function again in the rainfall. A circulation of warm air follows 

 the descending rain which percolates charged with many ingredients 

 for the sustentation of plant life. Water is heaviest at a tempe- 

 rature of 40 ; heat cannot pass downwards through water ; hence 

 the saturated ground can only be renovated by sun heat on its 

 surface, and the dense cold water should find its way from the sub- 

 soil by drainage, either natural or artificial. In Denmark they practise 

 a plan of double drainage ; a system of piping serves ordinary pur- 

 poses near the surface, but these drains communicate with a lower 

 system, which again is generally conducted to some natural water 

 storage, the outlets into which are guarded by sluices. In seasons 

 of drought these sluices may be closed and the water retained. The 

 soil there is generally light. The depth at which drain-pipes should 

 be laid, and the distances at which they recur, must be ruled by the 

 nature of the soil. In stiff clay they should be placed 3 ft. below 

 the surface, and 16 ft. apart ; in light soil they may be put at intervals 

 of 40 ft., but 4ft. deep. In non-porous soil, cuttings for the drains 

 should be filled to within 6 in. of the surface with porous material. 

 All main outfall drains should be laid with a minimum fall of I in 

 500, or 10 ft. 6 in. per mile. Collecting drains should have a minimum 

 fall of i in 250, or 21 ft. per mile. These should enter the main 

 ducts at an acute angle. In any system of drainage it is desirable 

 to make wells at convenient places, that may serve as catch-pits, 



