36 PHYSIOGRAPHY 



The reason why the water in a geyser tube is shot out, and at 

 intervals, while the water in an open kettle is not, is found in the 

 difference in the shape of the vessels holding the water. When 

 water is heated, it expands. When water is heated in a kettle, 

 that at the bottom rises readily (the motion is convection) to the top, 

 so that there is a nearly uniform temperature throughout. The 

 geyser tube is much deeper than the kettle, and in places its diameter 

 is probably small. The tube is also more or less crooked. Both its 

 smallness and its crookedness interfere with the rise of the water (by 

 convection) heated below, and the result is that water below the 

 surface is brought to the boiling temperature, under conditions 

 which do not allow the vapor to escape freely into the air. Hence 

 steam is formed in quantity below the surface, and by its great 

 expansion blows out the water above. 



If a stone or clod of earth, or almost any other solid object is 

 thrown into a geyser, its eruption may be hastened a little, because 

 such things interfere with the convection of the water in the tube. 

 They help to hold the hot water down where it is being heated, and 

 so help it to reach a boiling temperature at some point below the 

 surface, a little sooner than it would do otherwise. 



Geyser-water is constantly cooling the hot rock beneath the sur- 

 face, and in time the rock will cease to be hot enough to boil the 

 water. Geysers will then cease to exist, unless new supplies of lava 

 are forced up from below. In the Yellowstone Park, some geysers 

 have died out since the region became known, scarcely forty years 

 ago. New geysers, on the other hand, have come into existence 

 in the same region during this period. 



Artesian and Flowing Wells 



When the water in a well rises to or above the surface, the well 

 is said to flow. Flowing wells are not unlike springs whose waters 

 spout up as they issue. The chief difference between them is that 

 the opening in one case is natural, while in the other it was made by 

 man. Formerly, artesian wells were regarded as the same as flow- 

 ing wells. Nowadays, the name " artesian " is often applied to deep 

 wells, whether they flow or not. 



Fig. 23 illustrates the general conditions necessary for flowing 



