4H 



THE POPULAR SCIENCE MONTHLY. 



Iceland. 6. The temperature of the water in the basin was found to 

 be usually 170 to 180, and that in the tube to increase rapidly, 

 though not regularly, with depth. Moreover, the temperature, both 

 at the surface and at all depths, increased regularly as the time of 

 eruption approached. Just before the eruption it was, at the depth 

 of about forty-five feet, very near the boiling-point for that depth. 



1. It is well known that the boiling-point of water rises as the 

 pressure increases. This is shown in the adjoining table. 2. It fol- 

 lows from the above that if water be under 

 strong pressure, and at high temperature, 

 though below its boiling-point for that press- 

 ure, and the pressure be diminished sufficient- 

 ly, it will immediately flash into steam. 3. 

 Water heated beneath, if the circulation be 

 unimpeded, is very nearly the same tempera- 

 ture throughout. That it is never the same temperature precisely is 

 shown by the circulation itself, which is caused by difference of tem- 

 perature, producing difference in density. The phenomenon of sim- 

 mering is also a well-known evidence of this difference of temperature, 

 since it is produced by the collapse of steam-bubbles rising into the 

 cooler water above. 4. But if the circulation be impeded, as when 

 the water is contained in long, narrow, irregular tubes, and heated 

 with great rapidity, the temperature may be greater below than above 

 to any extent, and the boiling-point may be reached in the lower part 

 of the tube, while it is far from this point in the upper part. 



We will suppose a geyser to have a simple but irregular tube, with- 

 out a cave, heated below by volcanic fires, or by still hot volcanic 

 ejections. Now, we have already seen that the temperature of the 

 water in the tube increases rapidly with the depth, but is, at every 

 depth to which observation extends, short of the boiling-point for that 



depth. Let absciss a d (Fig. 8) repre- 

 sent depth in the tube, and also press- 

 ures ; and the corresponding temper- 

 ature be measured on the ordinate 

 If, then, a b, b c, c d, represent 



CT i-* 



1 Atmos. 



o o 



rt 



2 Atmos. 

 33.3 ft. 



3 Atmos. 

 66.6 ft. 



4 Atmos. 

 100 ft. 



a n. 



equal depths of thirty-three or more 

 feet, which is equal to one atmospheric 

 pressure, the curve e/passing through 

 210, 250, 275, and 293, at the hori- 

 zontal lines, representing one atmos- 

 phere, two atmospheres, three atmos- 

 pheres, etc., would correctly repre- 

 sent the increasing boiling-points as 

 we pass downward. We shall call 

 this line, ef, the curve of boiling-point. 

 The line a g commencing at the surface at 180, and gradually ap- 



// 



Pig. 8. 



