ROADS, BRIDGES AND DRAINS 59 



of the river which would be apt to be knocked to pieces and 

 swept away in times of flood by trees swept down the river. 

 The cost of suspension bridges is not very great. The chief 

 item of cost is usually their erection, as they require deep, wide 

 and very secure foundations of cement for the blocks which 

 have to bear the strain of the wire ropes. A good foundation 

 is absolutely essential, and without this they cannot be built up 

 on soft soils. 



There are some estates which, from their nature, require 

 few drains. These are estates planted upon land with 

 gentle slopes, off which heavy rainfall naturally flows away. 

 Their surface waters are readily got rid of, and but a few deep 

 drains are required to collect the surface waters together, before 

 they assume too great a volume, and conduct them peacefully 

 to some outlet from the estate. 



There are other estates, however, flat and low-lying, with 

 which good and efficient drainage is a matter of life and death. 



Estates with light sandy soils through which rain readily 

 percolates are seldom in need of much drainage. 



The water in the soil is lowered to the depth to which the 

 drains are excavated, if the drains really carry away the 

 water they receive, so that, instead of having constantly 

 stagnant water in the soil, a movement of both water and air 

 is established. The introduction of air which follows the 

 water drawn off in drains stimulates the whole depth of the 

 surface soil into activity. Plant-roots cannot possibly grow 

 without oxygen from the air. Hence, in a water-logged soil, 

 the roots are confined to shallow surface layers. Only after 

 drainage can the roots go down so far as the circulation of air 

 extends. When the air penetrates down, the bacteria start 

 actively upon their work of supplying nitrogen to the roots of 

 the trees. 



The constant presence of an over-supply of water in the 

 soil means death to the beneficial bacteria on which the roots 

 of the trees depend for their nitrogen nourishment. It also 

 means that the roots of the trees cannot breathe. With poorly- 

 developed, badly-nourished roots, trees cannot maintain a 

 healthy condition. 



Not only is this the case, but water-logging encourages 

 the activity of certain detrimental bacteria known as de- 



