T. A. Jaggar — Conditions affecting Geyser Eruption. 327 



voir, so that the water in the flask never reaches the boiling 

 point. If the water-level of the reservoir be maintained con- 

 stant, this circulation will continue indefinitely, and in such case 

 there will be a dome-shaped mass of hot water continually boil- 

 ing up and overflowing at the geyser's mouth, as in the case of 

 the Excelsior geyser. Now at this stage, if the water-level in 

 the reservoir be allowed to sink under the drain upon it, it may 

 fall to a level six inches below <?, without interrupting the con- 

 tinuous overflow; in other words, it may fall back to the o 

 level, and yet the geyser will continue to act as a boiling 

 spring, without entering into an eruptive phase. The cause of 

 this is to be found in the differential expansion of the water 

 noted above, and a convectional upflow which acts as a driving 

 power even against a reversed head, after overflow has once 

 been established.* The overflow tube t may at this stage be 

 led into the reservoir at the b level ; this establishes a perma- 

 nent circulation, the only loss being by evaporation. With 

 the diminution in pressure if the level of the reservoir sinks, 

 there is tendency toward diminished inflow of cooler water at 

 the supply-tube: this implies rise in temperature of the water 

 at the base of the geyser-column, which tends to augment both 

 volumetric expansion and convectional velocity. Hence there 

 is here a critical point where the hydrothermal and hydrostatic 

 forces are in very delicate equilibrium ; if the reservoir is low- 

 ered an inch, the overflow decreases, ebullition takes place 

 below, and an eruption of extraordinary violence takes place. 

 The same effect is at once produced by placing the glass stop- 

 per in the reservoir bottle and so checking the atmospheric 

 pressure. When the mouth of the geyser-tube is flush with 

 the bottom of the basin, an eruption may be induced by stop- 

 ping the overflow tube t and permitting the water-level to rise 

 in the basin, thus augmenting the pressure on the geyser 

 column. Eruptions once started will continue intermittently, 

 if the hydrostatic conditions are maintained constant: if, how- 

 ever, the water-level of the reservoir again rises to a point 

 where continuous overflow is possible at the geyser's mouth, 

 the eruptions will cease and a hot-spring phase will follow. 



3. Field application of the results of experiment. 



The two simple experiments described, when compared with 

 the facts of nature, account for the most essential variations 

 observed in the phenomena of geyser eruption. Both are 

 methods of draining the reservoir : the one continuous, the 

 other spasmodic. In the same way the geyser-springs drain 



*Such convection currents gain no momentum without overflow, hence at the 

 a level convection played no essential part in the phenomena observed. 



