ON DRAINING. 95 



small 0, vent-liole at the top of a cask suffices to enable us to 

 withdraw a great stream from it at the bottom, and every 

 one knows that the bulk of liquid discharg'ed in a given 

 time is in quantity precisely equal to the volume of air 

 which enters in the same time. The fact of rain-water 

 sinking- tlirough the soil is demonstrative of the permeability 

 of that soil to air, as ever}^ drop of water which falls from 

 the heavens must first displace an equal volume of air before 

 it can enter the soil ; the water would remain on the surface, 

 and never sink, if, by reason of its superior g-ravity, it did 

 not push aside the air in its descent, which it does until it 

 meets with some subterranean level where the earth is satu- 

 rated with the fluid, and the rain-water then comes to rest, 

 having- disturbed and displaced air throug-hout its Avhole 

 downward course. And by this action we are led to observe 

 one beautiful provision of nature for renewing- the constituent 

 air of the soil, and I regard it as an argument in favour of 

 deep, as compared with shallow drainage, that a greater 

 bulk of earth is thereby filled Avith air, and with frequently 

 renewed air. 



There are other equally beautifid processes incessantly 

 active to maintain a full supply, and fresh supplies of air in 

 the soil. The continual change of temperature in the soil, 

 and in the atmosphere reposing upon it, has its effect ; but 

 probably the most potent cause is the unceasing- appropria- 

 tion by plants, or manures, or soil, of some one or other of 

 the three gases of which the atmosphere is composed. A 

 renewal of the particular atmospheric gas consumed, whether 

 it be oxygen, nitrogen, or carbonic acid, must be nearly con- 

 sentaneous with its use, and is effected by the well-known 

 principle of the diffusion of gases, and without which neither 

 phmts nor animals could live. 



I have spoken of cess-pools as usefid and convenient breaks 

 in lines of drains, particidarly in the long- run of a main, or 

 where several lines of drains converge from two or more 

 directions in one common central point to an outfall. The 

 use of the cesspool in drainage is an old English practice ; I 

 have found it in several counties, both north, south, and 

 midland : it is usually constructed in brick. The specimens 

 now exhibited are made of large earthenware pipes nine 

 inches in diameter, with a flat tile or foot on which to place 

 them in the soil. This plan will be found advantageous and 

 cheap, as the foremnn drainer may fix his cess-pools without 

 needinij- bricks and mortar and a bricklaver. The holes for 



