and Suhterraiiean Water Storage. 67 



portion is far smaller. Thus the river Severn drains 3890 square 

 statute miles of mountain, hilly, and plain country, before it passes 

 Gloucester. At that point of its progress about twenty-three 

 parts in a hundred, or rather less than one-fourth of the whole 

 rainfall, passes down in the year. The river Thames at Staines 

 has drained 3086 square statute miles of hill and plain, and wide 

 valleys, and there discharges about thirty-three parts in a hundred, 

 or one-third of the rainfall of the district. The river Seine 

 was long- ago estimated by Arago to carry off about one-third of 

 the rainfall, and as the country it drains is not unlike that of the 

 Thames drainage area, the correspondence is striking and satis- 

 factory. In all these cases the country is partly under cultivation, 

 but much more so in the valley of the Thames and Seine than in 

 that of the Severn. In other countries similar results have been 

 obtained. Thus the Saone, a rapid stream draining much 

 mountain and uncultivated land far from the sea, and joining the 

 Rhone, discharges into that river nearly three-fifths of the esti- 

 mated total rainfall of the drainage area, while the Tiber is 

 believed to carry off as much as seven-tenths. The calculations 

 however, with regard to these rivers, are of doubtful accuracy. 

 In the case of the great river Mississippi and its numerous large 

 feeders, it is believed, from tolerably certain data, that only one- 

 fourth part of the total rainfall of the drainage area reaches the 

 gulf of Mexico. Thus in a general way we may consider that 

 from one-fourth to one-third part of the rain that falls runs 

 along over the surface to the ocean. 



The result of human labour and cultivation on the natural 

 drainage of a country is in all cases to increase the quantity of 

 water carried off, and to carry off the water more quickly. By 

 the clearing away of forest, and by all kinds of artificial drainage, 

 both these results are obtained ; and such works, though they 

 may at first seem simple agricultural improvements, have a 

 marked influence on the climate and physical features of the 

 country operated upon. When Fens, like those of the East of 

 England or Holland, are laid dry by lifting the water, when 

 straight cuts are made to relieve tortuous watercourses, when 

 by deep drainage we actually bring back to the surface water that 

 was already on its way into the rocks, the quantity of water that 

 reaches the sea is increased, and this extra quantity is abstracted 

 from that which would otherwise have entered the earth and circu- 

 lated through it. There are no data by which we can judge of the 

 exact difference, but in proportion as a district becomes drained 

 it is certain that all the causes alluded to must act with increasing 

 intensity, and may ultimately seriously interfere with the sub- 

 terranean storage. Let us then, in the next place, consider this 

 subject of storage a little more in detail. 



F 2 



