460 WATER SUPPLY AND SEWAGE DISPOSAL 



ground-water rises into the river at many points. Figure 281 

 also shows how this may be so. 



Many cities, in attempting to secure an adequate water 

 supply, have constructed what are known as INFILTRATION- 

 GALLERIES (Fig. 281). These are merely wells sunk beside 

 the rivers. Sometimes from the bottom of the wells long 

 horizontal galleries extend along beside the river or out be- 

 neath it. Each gallery is, of course, bricked up to prevent 

 the ground from caving into it. Other cities have buried 

 FILTER-CRIBS in the bottom of the stream itself. In many 

 cases it was expected that the river-water would find its way 

 into the galleries and cribs. Such has rarely been the case. 

 Repeated and frequent examinations of the water in such cases 

 by chemists have nearly always shown that the gallery or the 

 crib contains ground- water, not river- water. It is evident that 

 the gallery or the crib has intercepted the ground-water on its 

 way into the river. The general movement, then, of the ground- 

 water along a river bed is nearly always into the river from either 

 side and up into it from beneath. What forces the water up 

 into the river from beneath? The answer to this question will 

 be evident when we have studied the pressure of standing water. 

 (See Art. 574.) 



In some arid regions the flow of water along a river valley 

 is the reverse of that just described. For example, along the 

 Platte River in Colorado and western Nebraska the percola- 

 tion of water is generally from the river off to either side into 

 the soil. The river is fed by the melting of the mountain 

 snows and, as the water runs out on the arid plains, its sur- 

 face is higher than the general level of the water-plane. The 

 water of the river, therefore, soaks out into the soil and in the 

 dry season completely disappears. In the spring or early 

 summer the river is said "to come down" from the mountains. 

 Explain what is meant. 



GROUND-AIR 



638. Importance of Ground-air. Closely related to ground- 

 water and, more or less controlling its movements, is the GROUND- 



