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which the bubbles will force their way up through the water in the channel of 

 the spring without producing any disturbance in the pressure in the channel. This 

 is the case with the continually ebullient springs. But if the channel is at any 

 point so narrow that the gas bubbles can fill it entirely, then the water column 

 resting on the gas volumes below becomes shorter. The decrease of pressure thus 

 produced will give rise to a more intense ebullition and emission of gas from the 

 cavities and adjacent side channels. The current of gas bubbles through the channel 

 becomes therefore more intense, and increasingly displaces the water from the 

 channel. Consequently the emission of gas and the ebullition increase still more, 

 as the pressure of the water diminishes. 



At last the water columns barring the passage of the gas bubbles through the 

 channel become so few and so small, that the steam pressure from below can 

 easily overcome them. They are then forced through the channel with great speed, 

 and thrown to a considerable height when expelled. The eruption of the spring 

 is then in full activity. The length of the interval from the moment when the 

 first bubbles appear, until the eruption takes place, is to a great extent dependent 

 on the shape of the spring channel. 



In springs with a simple form of channel, the eruption may commence sud- 

 denly, almost simultaneously with the appearance of the first gas bubbles. This 

 is most readily explained by the experiment of forcing air up a narrow perpen- 

 dicular glass tube filled with water. The air pushes the water column before it 

 up the tube, and expels the water almost all at once. On the other hand, where 

 the spring channel is irregular, being in some parts very wide and in others very 

 narrow, some time may elapse before the water is thrown so high above the basin 

 that a real eruption may be said to have begun. 



The length and character of the eruptions also depend very much on the 

 shape of the channel, but in this respect the conditions under which evaporation 

 and ebullition take place down in the cavities is also of great importance. In the 

 interval between the eruptions, during which the spring is at rest, a large supply 

 of energy, in the form of highly healed water, is accumulated down in the cavities. 

 The duration of the eruptions seems to indicate that this reserve energy of the 

 spring has some resistance to overcome before it can properly develop itself. Of 

 course, some of the hot water is accumulated in recesses and side channels leading 

 to the principal cavities, and can only take effect gradually during the eruption, 

 because very probably it has first to pass through narrow channels into the prin- 

 cipal cavities. 



During the eruption itself it is possible that the passage of the spring exhala- 

 tions through the principal spring channel may frequently be hindered by small 

 water columns, formed in the bends or narrow passages of the channel by the 

 water which the vapour carries from below, or which is convej'ed to the principal 

 channel from the side channels. 



While the eruption is at its height these stoppages only last a very short 



