62 hot springs; steam- jets. 



to support the pressure from above. It therefore 

 is cooled and returns again to the liquid state, 

 just as you may see happen — a short time before 

 the contents begin to boil in a glass retort, filled 

 with water, and set over the fire — to the steam- 

 bubbles as they rise up from the bottom into the 

 upper part of the liquid. By this process the 

 combined heat of the steam is given up to the 

 cooler liquid around it, and gradually heats it until 

 the steam that follows is no longer condensed and 

 can therefore rise to a greater height. In this manner 

 the heat from the depths of the earth passes gra- 

 dually towards and even up to the surface; the 

 temperature however becomes less and less as it 

 mounts upwards, since the water cannot take any 

 higher degree of heat than that by which, under 

 the pressure which it has to bear, the formation of 

 steam is kept up. For instance, at the depth of 

 ten thousand feet, water may be raised, by the 

 continual influence of the rising steam, up to 

 353° C. (667°'4 P.), but cannot be heated above 

 this temperature. At a depth of thirty-four feet 

 it cannot be heated above 121° 0. (249°'8 I\), 

 nor at the surface above 100° C. (212° I\) As 

 soon then as this temperature is reached, the whole 

 excess of heat of the hot water, that comes up from 

 below, is employed in forming steam, and the liquid 

 at that part begins to boil. 



The following table shows the amount of heat 



