38 The Drainage of Fens and Loiv Lands. 



The admission of air to the soil not only improves its 

 texture, but also raises the temperature, and supplies nourish- 

 ment to the plants. There is no doubt that a well-drained 

 and consequently a well-aerated soil, requires much less 

 manure than one that is sodden with water. There are many 

 mineral and organic substances in all soils which remain 

 dormant and useless to vegetation until decomposed by the 

 action of the atmosphere ; there are also many salts which 

 are unaffected by the water in the ground, but which, on 

 exposure to the air, are immediately set free and dissolved, 

 and carried to the roots of the plants. An excess of water 

 will thus neutralise the chemical decomposition of the sub- 

 stances contained in the manure laid on the fields, and which 

 largely supply food to vegetation* Drainage is as useful in 

 promoting the circulation of atmospheric air as in removing 

 the superabundance of moisture ; for if the pores in the soil 

 are emptied of water, it is evident that their place must be 

 supplied with air ; and as the effect of drainage is, by me- 

 chanically improving the texture of the soil, to increase the 

 number of these crevices, so it also increases the circulation of 

 the air, which passes through the soil to the drains, and along 

 them to their outlets, thus keeping up a constant supply of 

 fresh air. 



Temperature of Drained Land.— The temperature of 

 the atmosphere attains its maximum on an average of seasons 

 about the middle of July — ^the cold period attaining its maxi- 

 mum about the middle of January. The heat that is given 

 out in the summer is absorbed by the earth, and gradually 

 finds its way downwards until it reaches a depth beyond 

 which, speaking generally, the temperature of the soil is not 

 affected by the heat of summer or the cold of winter. This 

 depth is found to vary from 50 feet to 100 feet below the 

 surface, the variation of temperature between winter and 

 summer being only 3 degrees at 24 feet below the surface, the 

 mean variation of the atmosphere being, on the surface, nearly 



