116 



REMARKS ON DRAINING. 



operations are, however, intimately connected ; for if the former 

 were properly understood and practised, less draining- would be 

 necessary. If we detain for local use the water which falls on 

 the higher grounds, we, in effect, render the lower grounds drier 

 than they otherwise would naturally be, and to an extent that, 

 in some cases, might render draining unnecessary, or at least 

 would greatly modify the operation. 



It has been stated above that trees usually require all the rain 

 which falls on the space of ground they occupy. It may, there- 

 fore, be concluded that ground for them requires no drainage if 

 not liable to an influx of moisture from adjoining grounds. This 

 conclusion is correct in some cases, but in others, however para- 

 doxical the assertion may appear, it is not. 



Fruit-trees do not thrive well in any soil that is saturated with 

 water. Some soils are naturally so retentive, that comparatively 

 little rain is sufficient to maintain them in a saturated state — less 

 even than the trees would require if thriving in a porous soil. 

 Water is not in itself poisonous to plants ; the evils which it pro- 

 duces when the soil is saturated with it arise chiefly from its 

 mechanical properties, pressure and specific gravity. 



In order to become aware of the pressure of w r ater on any 

 body immersed in saturated soil, we may take a box 2 feet square 

 and 15 inches deep : place in the middle of it an empty box, 1 foot 

 square, 13 inches deep, and weighing, say 51bs. ; then fill in the 

 space round it with soil to the depth of a foot ; place in the 

 empty box an elastic ball or small bladder distended with air ; 

 suspend over this a quarter of a hundredweight so as to nearly 

 touch the ball. This arrangement being made, let the whole of 

 the soil be slowly moistened throughout by means of a watering- 

 pot with a tolerably fine rose. When this has been effected, a 

 little more water will cause saturation. The soil may be con- 

 sidered completely saturated when the water begins to stand 

 above it. Then a pressure of 62% lbs., the weight of a cubic 

 foot of water, will be exerted on the bottom of the empty box in 

 which the ball was placed ; the box and ball will, of course, be 

 forced upwards, the ball will be squeezed against the 28 lb, 

 weight till it bursts, unless strong enough to support that 

 weight. In fact, instead of 28 lbs. the box would require to be 

 loaded with 57^ lbs. in order to prevent its being raised up ; or 

 if its bottom were composed of a multitude of air-cells, they, 

 collectively, would be subjected to a pressure of 62^ lbs. the 

 moment saturation takes place to the height of a foot above 

 them. A root placed in a similar position would have the same 

 proportion of pressure on all parts of its surface with which the 

 water comes in contact, and just so much more than it would 

 have were those parts in contact only with air in the pores of the 



