THE TRAVELS OF THE UNDERGROUND WATER 193 



boiling point, but the expansion of the steam within the mass will 

 elevate the upper layers of the water into zones where the boiling 

 points are lower, and thus bring about a sudden and violent ebul- 

 lition of all these upper portions. Thus is explained the almost 

 universal observation that just before geysers erupt the hot water 

 rises in the bowls and generally overflows them. 



The water ejected from the geyser is considerably cooled in the 

 air, and after its return to the tube must be again heated by the 

 ascending vapors before another eruption can 

 occur. The measure of the cooling, the time 

 necessary to fill the tube, and the supply of 

 rising steam, all play a part in fixing the period 

 which separates consecutive eruptions. If the 

 top of the tube be narrowed from its average 

 caliber, as is commonly observed to be true of 

 the geysers within the Yellowstone National 

 Park, the escape of the steam is further hin- 

 dered, and frequent geyser eruption promoted. 



An artificial geyser for demonstration of the 

 phenomenon in the lecture room is represented 

 in Fig. 203. The cut has been prepared from 

 a photograph of an apparatus designed by 

 Professor B. W. Snow of the University of 

 Wisconsin. In this design the tube is con- 

 tracted so as to have a top diameter one fourth 

 only of what it is at the bottom, where heat is 

 directly applied by multiple Bunsen lamps. 

 The water once sufficiently heated, this arti- 

 ficial geyser erupts at regular intervals of time 

 which are dependent upon the dimensions of 

 the apparatus and the quantity of heat applied. 



In case of natural geysers a considerable 

 quantity of heat escapes between eruptions in 

 steam which issues quietly from the bowl of 

 the geyser. If this heat be retained by plugging the mouth 

 of the tube with a barrowful of turf, as is sometimes done 

 with the geyser Strokr in Iceland, eruption is promoted and so 

 takes place earlier. Another method of securing the same result 

 is to increase the viscosity of the water through the addition of 



FIG. 203. Apparatus 

 for simulating gey- 

 ser action in the lec- 

 ture room (by cour- 

 tesy of Professor B. 

 W. Snow). 



