50 GROUND-WATER 



be secured by a definite case. Suppose a geyser-tube full of water 

 and heated at its lower end. As the water is heated below, con- 

 vection tends to distribute the heat throughout the column of water 

 above. If convection were free and the tube short, the result 

 would be a boiling spring; but if the tube is long, and especially if 



Fig. 29. Giant Geyser, Yellowstone National Park. (Wineman.) 



convection is impeded, the water at some level below the surface 

 may be brought to the boiling-point earlier than at the top. If 

 even a little water in the lower part of the tube is converted into 

 steam, the steam will raise the column of water above, and it will 

 overflow. The overflow relieves the pressure on all parts of the 

 column of water below the surface. If before the overflow there 

 was any considerable volume of water essentially ready to boil, 

 the relief of pressure following the overflow might allow it to be 

 converted into steam suddenly, and the sudden conversion of a 

 considerable quantity of water into steam would cause the eruption 

 of all the water above it (Fig. 29). The height to which the water 



