=Movement of ground-water.=[96]—Ground-water is in more or less continual movement. If all the water be pumped out of a well it soon fills up again to its normal level by inflow from all sides. Springs and flowing wells also demonstrate the movement of ground-water. Near the surface the movement of ground-water is primarily downward if the medium through which it passes is equally permeable in all directions; but so soon as the descending water reaches the water surface, its descent is checked and its movement is partly lateral. The commonest sort of movement of ground-water is that exemplified as the water sinks beneath the surface, namely, slow percolation through the pores and cracks of the soil and rock. Ground-water is not generally organized into definite streams, though underground streams, mostly small, are sometimes seen in caves and crevices, and sometimes issue as springs. Most underground streams which issue as springs probably have definite channels for short distances only before they issue. It is probable that ground-water frequently flows in considerable quantity along somewhat definite planes, without having open channels. Thus every porous bed of rock is likely to serve as the pathway along which subterranean drainage passes. This is especially true where the porous bed is underlain by an impervious one. The “reservoirs” from which artesian wells draw their supply are not usually streams or lakes, but porous beds of rock through which abundant water passes. As the supply is drawn off at one point, it is renewed by water entering elsewhere. Since the freedom of movement of ground-water is notably influenced by the porosity of the rock, and since the rock is, on the average, most porous and the pores largest near the surface, the movement of ground-water is, on the average, greatest near the surface, and least at its lower limit. In general the decrease of movement is much more rapid than the decrease in the size of the pores. It follows that while the upper part of the ground-water, especially that above ground-water level, moves somewhat freely, the lower part moves much more slowly. It is probable, indeed, that the movement in the lower part of the subterranean hydrosphere is extremely slight.