LETTER V. 



HOT SPRINGS AND JETS OF STEAM. 



Besides the effect of water-pressure in siphon- 

 shaped clefts in the earth, winch is by far the 

 most general cause of the formation of springs, 

 there is another agency concerned in effects of this 

 kind. Tins is the expansive force, or pressure, of 

 gaseous bodies, especially of carbonic acid and of 

 steam; tins kind of pressure also acts sometimes 

 with enormous power in forcing water to flow 

 witlhn the earth, and to spout up above its surface. 



I have already pointed out to you that, at a depth 

 of about ten thousand feet, water must take the boil- 

 ing temperature by the internal heat of the earth. 

 But it by no means follows from this that water 

 must be there actually boiling. For the process 

 of boiling, that is the rising of bubbles of vapour 

 from within the water, implies (as we learn from 

 physicists) as a necessary condition, that the pres- 

 sure on the surface of the water must be less, or 

 at any rate not greater, than the counterpressure 

 of the expanding steam. Now it may be con- 

 sidered as generally true, that, however deep the 

 clefts in the earth may be, they must become quite 

 filled with water in a short time. We may there- 



