AGRICULTURAL CIIEMISTRY. 183 



improvement of cold soils, devised what, under the name of Thorough 

 Drainage, has become one of the most useful appliances in cultiva- 

 tion. 



Thorough drainage consists essentially in constructing underground 

 channels, sufficient in number and size, for the removal of surplus 

 water down to a certain depth. A clay field, for example, has a 

 system of parallel ditches dug in it, three or four feet in depth, and 

 sixteen to thirty feet apart. These have such an inclination, and so 

 connect with cross or main ditches, as to give the water that may 

 collect in them a ready discharge. The bottoms of the ditches are 

 then filled with small stones to the depth of about one foot, or have 

 carefully laid in them a pipe of baked clay, (drain tile,) one to three 

 inches in diameter, and are thereupon filled up with earth. These 

 channels at once discharge the water of rains and melting snows when 

 the soil is sufficiently porous; and if at first, as happens with clays, 

 the soil is too retentive to allow the ready removal of water, this evil 

 mends itself in a year or two. We know that a mass of clay exposed 

 to the air in dry weather gradually dries off superficially, and ap- 

 pears full of minute fissures or larger rifts. In time it becomes 

 entirely friable; and if water be poured on it the liquid, for the most 

 part, rapidly filters through. It is only by a prolonged immersion in 

 water that the dried clay absorbs so much of it as to become tenacious 

 and plastic again. The under drains are the effectual means of drying 

 out the clay soil to such a point that excess of water flows oif without 

 hindrance, and they are no less effectual in preventing the recurrence 

 of a too retentive state. 



The fact that we are in possession of extended treatises on drain- 

 age, renders it unnecessary to do more here than to allude to some 

 of the more striking results of this sj'stem which have been observed 

 in practice, and to indicate their scientific explanation. 



One of the most important effects of thorough drainage consists in 

 tempering the extremes of moisture and dryness, of heat and cold, 

 so that a drained soil is dryer in the wet seasons and moister in the 

 dry seasons' — is warmer in cold weather, and cooler in hot weather, 

 than an undrained soil. 



The result of the rapid removal of surplus water on the soil is such, 

 as enables it to be tilled from two to four weeks earlier in the spring 

 than might otherwise happen, a gain which, in cold climates or back- 

 ward spring-times, is often the saving of a crop. 



The vast mass of water that is thus removed without evaporation 

 corresponds to a large increase in the amount of heat which may 

 accumulate in the soil, an increase that is not only perceived in the 

 rapid growth of vegetation after the ground is prepared for seed, but 

 also is manifest in the earlier melting of snows. The official inquiries 

 of the Royal College of Rural Economy of Prussia show that the 

 snow in that country thaws away on the average one week earlier on 

 drained than on contiguous undrained land. 



It is said that Smith, of Deanston, was led to his study of drainage 

 by an observation made on ridged fields. From time immemorial it 

 has been a custom in some countries, especially in those overrun by 



