82 Rainfall, Natural Drainage, 



But, after all, disforesting is only the commencement of the 

 change. Each kind of cultivation involves some peculiar result 

 of its own, for as a country becomes thickly peopled nature is 

 made to bend in various ways to human convenience. First, 

 there is the general drainage of swamps and bogs to render the 

 country healthy and habitable, and then follow improvements in 

 the course of the streams to confine them within definite channels 

 that shall run as quickly as possible to the sea. By thus de- 

 creasing the distance run the erosive power of the streams is 

 increased, and therefore the conveying power of the water, so 

 that one of the results of the clearing of a mountain-side may be 

 the extension of a coast-line towards the ocean. Another result 

 may be, as in the case of the river Po, the gradual elevation of 

 the bed of the stream till its waters are carried between banks at 

 a level higher than that of the surrounding country. 



In like manner the drainage of shallow pools helps to increase 

 the mechanical effect of streams, while artificial embankments 

 limit and divert the action of the sea, recovering tracts of land 

 subject to tidal overflow, and converting them ultimately into 

 fields and gardens. It must be evident that the evaporation that 

 once acted over almost the whole surface of the land is now 

 reduced to the narrow courses of the streams except immediately 

 after heavy rains, and that the quantity of rain absorbed into the 

 earth must be much smaller now that the surface is dry than 

 when it was permanently moist. In this way, therefore, two 

 direct and important results of the introduction of civilized man 

 are at once recognized. It is true that in each particular case the 

 calculable difference may be small, but when the whole surface 

 is affected it is impossible that it should not be important. One 

 thing also leads to another. The diminution of mist arising 

 from permanent moisture on the surface increases greatly the 

 radiation from the surface, and therefore the evaporation. The 

 quality of the soil is thus altered by mechanical treatment, and 

 the moisture needed is more rapidly absorbed and utilised by 

 miscellaneous crops than by forest-trees. 



In every way the cultivation of the soil has a tendency to 

 modify the proportion of rainfall that passes into the earth. It 

 tends to increase this proportion by inducing in summer a greater 

 action both of the sun and air in drying, and therefore cracking the 

 surface, and during cold weather by exposing the rock more fre- 

 quently to alternate expansions and contractions. On the other 

 hand, it tends to diminish the proportion by running the water 

 more rapidly from the surface and leaving a smaller quantity to 

 soak into the strata. These are direct results. Indirectly, culti- 

 vation, even without drainage, by rendering the air more clear 

 during fine weather, and by increasing both the hourly and daily 



