AMERICAN INSTITUTE. 421 



dissolved out and brought to the surface from witliin the hill 

 such salts as rendered it hard. The extent of this experiment 

 and the time during which it continued, leave no doubt that the 

 water under certain very common circumstances carries no 

 organic matter which may be dissolved in it below a foot or so in 

 the soil, while it dissolves and brings to the surface continually 

 soluble substances from within the earth. 



The water of irrigation is merely a substitute for rain, and con- 

 sequently irrigation is most necessary and of course furnishes the 

 most striking results on such soils and such crops as feel drouth 

 soonest. As the substances that compose plants must at first 

 enter into their composition in a soluble condition, and as the 

 solution from which they are revived by the action of the sun on 

 plant tissue is exceedingly dilute, it follows that the growth of 

 the plant is principally governed by the supply of moisture . It 

 is the principal province of the laborious processes of tillage to 

 retain the natural supply of water and furnish it to the plant as 

 it is needed during the action of the sun. In the same manner 

 that a cloth wrung is freed from moisture and refuses to absorb 

 so does a hard soil. The bed of a turnpike road is sooner dry 

 than the neighboring plowed field. The surface of the earth 

 stirred by the plow to the depth of six inches will absorb and 

 retain one or two inches of rain which will give growing plants 

 a fair supply of water for ten days or two weeks of exposure to a 

 bright sun. By increasing the depth of tillage to twelve inches 

 the risk of a longer drouth is avoided and a greater aggregate 

 growth is secured, as there are less extremes of variation. It is 

 in this way that deep tillage of land seems to effect so much 

 benefit and indeed it is extremely difficult to account for the well 

 known benefits of frequently stirring the soil on any purely 

 chemical hypothesis. 



The supply of rain to the different countries of the earth, when 

 not interfered with by local causes, such as ranges of mountains, 

 will be found to correspond to the intensity of sunshine, and the 

 same is generally true of any given place for the diiferent sea- 

 sons of the year. Thus, while we have some thirty-five inches of 

 rain annually, the average fall in tropical countries is over a hun- 

 dred, and two-thirds of our thirty-five inches fall during the 

 hottest third of our year. 



In countries that are thoroughly cultivated the greatest part 

 of the rain that falls never passes into the earth more than a foot, 

 being absorbed and retained to be exhaled again by the growing 



[Am. Inst.] 28 



