34 PHYSIOGRAPHY 



through the porous layer 6, under pressure, until it reaches a crack 

 which leads up to the surface. If the crack is open enough to afford 

 a passageway, the water will follow it up to the surface, as at s'. 

 In such a situation there will be a spring only when the opening is 



d 



Fig. 22. Diagram to illustrate two types of springs, as explained in text. 

 The type represented by s is more common than the other. 



lower than the water surface in the layer of rock which carries the 

 water. This sort of a spring is similar to a flowing well in principle, 

 but in the latter case the opening is made by man. 



Temperature. The temperature of spring-water is very vari- 

 able. Most springs seem cold in warm weather, and there is a 

 popular impression that they are cooler in summer than in winter; 

 but this is not the case. The impression arises from the fact that 

 the water is much cooler than the air in summer, and so seems cold, 

 while in winter, the water may be warmer than the air, and so seems 

 less cold than in summer. Springs whose waters come from great 

 depths vary little in temperature during the year, while those whose 

 sources are but little below the surface are colder in winter than in 

 summer. The reason is, that the warmth of summer and the cold 

 of winter are most extreme at the surface, and become less with in- 

 creasing depth. Below 50 or 60 feet, in middle latitudes, the tem- 

 perature does not change much with the seasons, and springs which 

 draw their water from depths greater than 50 or 60 feet vary little 

 in temperature. 



Some springs are warm, and a few are hot. Where spring-water 

 is hot, it is commonly because it has been in contact with lava which 

 came up from greater depths so recently that it has not yet become 

 cold. There are more than 3,000 hot springs in the Yellowstone 

 National Park. 



Mineral and medicinal springs. All spring-water has some 

 mineral matter in solution; but a spring is not commonly called 

 a mineral spring unless it contains (1) much mineral matter, (2) 

 mineral matter which is unusual in spring-water, or (3) mineral 



