32 GROUND-WATER 



is slow. With the spreading, the surface of the water in the sand 

 sinks, and sinks fastest at the center where it is highest. If no 

 water were added, the surface of the water in the hill would, in 



Fig. 20. Diagram showing how rain-water, falling in one place, may flow under- 

 ground to another and there be brought to the surface. The layer a is porous, and 

 water entering it in the mountains follows it to the plain. 



time, sink nearly to the level of the water in the surrounding land; 

 but at every stage preceding the last, the surface of the water would 

 be higher beneath the summit of the hill than elsewhere, though 



Fig. 21. Diagram illustrating the position of the ground- water surface (the 

 dotted line) in a region of undulatory topography. 



farther from the surface. In regions of even moderate precipitation 

 the water-surface beneath the hills rarely sinks to the level of that 

 in the lowlands about them, before it is raised by further rains. 



The water-surface beneath lowlands also sinks. Some of the 

 water finds its way into valleys, some of it sinks to greater depths, 

 and some of it evaporates; but since the water-surface beneath the 

 elevations sinks more rapidly than that beneath the lowlands, the 

 two approach a common level. Their difference will be least at 

 the end of a long drought, and greatest just after heavy rains. 



Depth to which ground-water sinks. The depth to which 

 ground-water sinks has not been determined by observation. The 

 deepest excavations are but little more than a mile deep, and at 

 this depth the limit of water is not reached. There is a popular 

 belief that water sinks until it reaches a temperature sufficient to 

 convert it into steam; but except in places where hot lava lies near 

 the surface, this belief does not appear to be well founded. Its 

 descent probably is stopped in quite another way. 



Water descends through the pores and cracks of soil and rock, 



