60 HOT SPRINGS; STEAM- JETS. 



fore assume, that the water, which has found its 

 way down to the depth I have mentioned, has to 

 bear the load of a column of water ten thousand 

 feet high, that is to say a pressure of ^ or about 

 two hundred and ninety-four atmospheres (taking 

 the pressure of the atmosphere as equal to that of 

 a water-column thirty-four feet in height). But 

 steam of the temperature of 100° C. (212° P.) can 

 only sustain a pressure of one atmosphere. The 

 water therefore in these deep reservoirs must behave 

 exactly like water, which is exposed in a perfectly 

 closed boiler to a temperature of 100° and up- 

 wards. It remains quite liquid, notwithstanding 

 its efforts to form vapour, while the force, with 

 which it presses on the walls of the vessel, in- 

 creases as the temperature rises. Experimental 

 inquiries concerning steam have shown that its 

 elasticity, that is its power of balancing external 

 resistances increases much more rapidly than its 

 temperature. We know for instance that ice-cold 

 water, and even ice itself, gives off vapour, and that 

 this is the reason why, when placed in open vessels, 

 it is gradually dissipated into the air — that it evapo- 

 rates. However the elasticity, or expansive force, 

 of this vapour is very slight. To enable water to 

 give off vapour which can resist the pressure of the 

 atmosphere, a temperature of 100° C (212° P.) is 

 necessary. Vapour rising from water heated to 

 200° C. (392° F.)— a temperature which prevails 



