THE DRAINAGE OF THE FARM. 4(33 



they are half an inch in diameter the water will be observed to have a. 



' i tly concave surface, that on the outer edges being attracted up the 



s of the glass a little. With tubes much smaller, this peculiarity will 

 be still more distinctly seen ; and with those that have so small internal 

 openings, that they only admit a fine wire, it will be found that the water 

 will not even run through them, if they are in short length. The whole 

 opening is charged with liquid ; but, in consequence of the attracting or 

 retaining power of the glass walls, it is in a state of rest ; and to overcome 

 thi- ly natural means, we require to add pressure from above, 



by increasing the length of the tube, and filling it to the surface. From 

 the nature of the air canals which exist in the soil, we are justified in 

 comparing them to an extensive series of vertical and crooked or diagon- 

 ally arranged tubes bundled together, with their mouths at the surface 

 and their exit openings a little above the level of the drainage channels. 

 If the drains are shallow, these tubes must of course be short; and 

 the head pressure being less than if they were long, they will necessarily 

 evacuate the water more slowly than if they were longer. This theory is 

 completely borne out by practical observation ; for, in most cases (other 

 circumstances being equal), deep drains run sooner and faster after rain 



it. shallow ones. And, as the pores or tubes of dense soils are more 

 minute than those of light or open land, and will therefore require to have 



eased head pressure to promote the free circulation of water, the theo- 

 retical reason comes out, how it is that drains require ,to be deeper in 

 clayey ground than in any other kind of soil. 



often meet with practical farmers who assert that, if four-feet 

 drains are adopted on heavy soils, the rain water falling upon the surface 

 will fail to reach them. They suppose that most clays are so impervious > 

 that water cannot percolate through a stratum four feet thick. Now, I 



lit at once, that there are clays to be found, which are so plastic and so 



.hen trodden upon or pressed in any way, that even an inch in 



tliir. ill hold a pool of water for ft considerable time. I have seen a 



ntity of water .standing in a clay furrow for weeks together, witli 



11. -lit pipe-til-- drain underneath, having only six inches of CON 



found its way into tin- drain from the pool above; and, had 



ro been a layer of clay but two inrli.-s thi<-k, I believe the iv-ult would 

 have be.-n the same. What inference, then, are we to draw from : 

 fact? Are we to argue that drains six inehes deep are not shallow en. 



rcolation or water from th of dense clays ? 



> deep in some cases, what other depth can we 



make them, to injure their ell'n-ieiK-y > We . that, if 



