THE WORK OF GROUND-WATER. 



237 



vection tends to distribute the heat throughout the cohimn of water 

 above. If convection were free, and the tube short, the result would 

 be a boiling spring; but if the 

 tube is long, and especially if 

 convection is impeded, the water 

 at some level below the surface 

 may be brought to the boiUng- 

 point earlier than that at the top. 

 Under these circumstances if even 

 a little water in the lower part of 

 the tube is converted into steam, 

 the steam will raise the column of 

 water above, and it mil overflow. 

 The overflow relieves the pressure 

 on all parts of the column of water 

 below the surface. If before the 

 overflow there was any consider- 

 able volume of water essentially 

 ready to boil, the relief of pressure 

 following the overflow might allow 

 it to be converted into steam 

 suddenly, and the sudden conver- 

 sion of any considerable quantity 

 of water into steam would cause 

 the eruption of all the water above it (Figs. 213 and 214). The height 

 to which the water would be thrown would depend upon the amount of 

 steam^ the size and straightness of the tube, etc. 



It is clear that everything which impedes convection in the geyser 

 tube will hasten the period of eruption, since impeded circulation will 

 have the effect of holding the heat do^vn, and so of bringing the Avater 

 at some level below the top more quickly to the boiling-point. It 

 follows that anything which chokes up the. tube, or which increases 

 the viscosity of the water, or its surface tension, would hasten an 

 eruption.! , 



Geysers often build up crater-like basins or cones (Figs. 214 to 217) 



Fig. 213.— 'Old Faithful" in eruption. 



^Weed. Ninth Ann. Rept. U. S. Geol. Surv., pp. 613-76, and Am. Jour. Sci., 

 Vol. XXXVII, 1889, pp. 351-59. 



