50 PHYSICAL GEOGRAPHY. CHAP. 



6, 7, and 8, the instruments being carefully placed in the shade ; 

 on the other hand, there are many days when only a small fraction 

 of a degree distinguished the dry from the wet bulb. 



Sites of population. Supply of water. The earliest settlements 

 among a pastoral people must have been near springs of water, 

 or on the course of rivers, or by the side of lakes : nor have the 

 changed circumstances of society at all diminished this primary 

 dependence of man and his cattle upon the supply of wholesome 

 water. On the contrary, society has grown fastidious, and chooses 

 to bring its supplies from distant sources, even to the banks of the 

 rivers on which towns are nourishing. One of the great questions 

 for the Roman administration has been revived in our day, under 

 far more difficult conditions, and the springs of all the rivers of 

 England are upon their trial in the great struggle for the pure 

 element. If it could be proved that the fall of rain is less than 

 it formerly was, and that artificial circumstances of any kind may 

 influence the quantity, the extent of woods, the system of drainage, 

 and the kind of land cultivation may be brought into the com- 

 plicated problem. 



The effects of artificial drainage are twofold. Surface drains, 

 straighter and clearer than the old ditches direct cuts instead of 

 winding brook-channels -must carry off rain and snow-water 

 more rapidly than was formerly the case. Thus some water may be 

 abstracted from springs, but not much. Thus floods must happen 

 more quickly after rain, and may gather in greater force in the 

 broad expanses of the valleys. But deep drains, which draw down 

 surface-water, and cause it to be slowly discharged in almost con- 

 tinuous small underground rills, have an opposite effect; they 

 augment the subterranean streams, and feed, as by new springs, 

 the permanent flow of the rivers. 



In any case the basin of the Thames may for a long period be 

 the only source of supply to London ; and as, on every stream which 

 contributes its share to this river, every day is increasing the local 

 demand for water, the prospect of a sufficient current being ab- 

 stracted from the main stream becomes less and less encouraging. 



Almost all the villages, and a great part of the detached farms 

 in the Cotswold country, are seated by springs of clear, but usually 

 rather hard, water. The hardness is commonly from carbonate 

 of lime. Carbonate of iron occurs not rarely, and sulphate of lime 



