THE MECHANICS OF INTERMITTENT SPRINGS. 373 



the walls of which are formed of thick stone, free from penetrating 

 clefts. Water-ways entering it from above and from the sides bring 

 in cool waters ; while other streams bring up from below water heated 

 to above the boiling-point, or steam from the volcanic foci. The 

 inferior conducting power of the rock-masses, inclosing the chamber 



Fig. 2. Mackenzie's Theory op Eruption. 



and the stream-channels, prevents any material diminution of the tem- 

 perature of the steam and the water, so that the chamber, b, becomes 

 filled with hot water and steam. By virtue of its levity, the latter 

 collects in the upper part of the chamber, while the water covers the 

 bottom, and at the same time, as it is added to by the constant inflow 

 from the canals which enter the chamber, b, rises and shuts up the tube 

 at a. The steam is thus deprived of an outlet, and, since it is con- 

 tinually compressed into a smaller space by the constantly increasing 

 mass of water, is added to by the entrance of new steam, and is heated 

 to a higher temperature by the heat brought in with the hot water 

 that keeps flowing in ; it on its side exercises upon the surface.of the 

 water in the chamber a pressure that, increasing every moment, 

 gradually raises the water in the tube, c, and causes an overflow over 

 the rim of the basin, d. Finally, the pressure of the inclosed steam 

 becomes so powerful as to overcome the absolute weight of the mass 

 of water in- the tube, c, and throw it up strongly and suddenly in 

 fountain-like spouts. After the steam in b has relieved itself, and the 

 pressure on the water has thereby been diminished, a becomes again 

 closed up by the rushing back of the water from the tube and the 

 flow of water from the chamber, and the conditions requisite to an- 

 other eruption are produced. The temperature of the water in the 

 chamber and in the tube rises continually between one eruption and 

 another, and reaches in many places the degree at which steam is 

 formed. This is the case in the lower part of the tube at a, where, 

 the water nearer the chamber having a higher temperature than in 

 other parts of c, the steam rises in c and causes the bubblings which 

 appear in the basin at intervals of from one and a half to two hours. 

 This theory, which supposes a subterranean cavity acting as a kind 



