66 hot springs; steam-jets. 



moisture is always oozing, quickly and easily 

 dries up, and becomes coated with a crust of 

 flinty matter. A little farther off, where the water 

 spreads itself over the ground around the spring, 

 this crust increases in thickness as the evaporating 

 surface increases. By this means the ground is 

 gradually raised on one side of the basin, and so 

 opposes an obstacle to the escape of the water in 

 that direction ; the overflow is thus transferred to 

 another side which may be a little lower, where the 

 sinter-laying process begins again and goes on, 

 until the change in the level of the bottom causes 

 another change in the place of discharge. Since 

 the basin from which the spring runs receives 

 within itself no share of this incrustation, it keeps 

 gradually growing upwards, while a hillock of flint- 

 sinter is built up around it till it becomes a deep 

 tube. This tube having a comparatively small bore, 

 and being filled with a column of water strongly 

 heated below, and not escaping too slowly, joins 

 in itself all the conditions of a constantly flowing 

 spring, as is observed in many places in Iceland. 

 If now the rising mass of water, ever renewed 

 from below, is heated, in the depth of its self-built 

 shaft, to above 100° C. (212° F.)i it must, as it 

 mounts, suffer a continual lowering of its tempe- 

 rature always corresponding to the diminished pres- 

 sure (according to the table, p. 63), and at last 

 at the surface cannot be hotter than 100° C. 



