"'He (Krug Von Nidda) takes it for granted that these hot springs derive their temperature from the aqueous vapors rising from below. When these vapors are able to rise freely in a continued column the water at the different depths must have a constant temperature equal to that at which water would boil under the pressure existing at the respective depths; hence the constant ebullition of the permanent springs and their boiling heat. If, on the other hand, the vapors be prevented by the complicated windings of its channels from rising to the surface; if, for example, they be arrested in caverns, the temperature in the upper layers of water must necessarily become reduced, because a large quantity of it is lost by evaporation at the surface, which cannot be replaced from below. And any circulation of the layers of water at different temperatures, by reason of their unequal specific gravities, seems to be very much interrupted by the narrowness and sinuousity of the passage. The intermitting springs of Iceland are probably caused by the existence of caverns, in which the vapor is retained by the pressure of the column of water in the channel which leads to the surface. Here this vapor collects, and presses the water in the cavern downward until its elastic force becomes sufficiently great to effect a passage through the column of water which confines it. The violent escape of the vapor causes the thunder-like subterranean sound and the trembling of the earth which precedes each eruption. The vapors do not appear at the surface until they have heated the water to their own temperature. "'When so much vapor has escaped that the expansive force of that which remains has become less than the pressure of the confining column of water, tranquility is restored, and this lasts until such a quantity of vapor is again collected as to produce a fresh eruption. The spouting of the spring is therefore repeated at intervals, depending on the capacity of the cavern, the height of the column of water, and the heat generated below.'" Dr. Peal continues: "Bishof says that the eruptions of the Geyser and Strockr agree exactly with this explanation and he accounts for the two distinct classes of eruption observed in the Geyser as follows: "'The two distinct classes of eruption in the geyser which we have already mentioned seem to be attributable to two different cavities. A small cavity fills quicker, and, therefore, empties itself more frequently; a larger one fills slower, empties itself seldomer, but with greater violence.'" Bunsen's theory is the next considered and is somewhat similar to Bischof's but with notable differences. After taking temperatures at different points in the Geyser tube his first conclusions are that: (1) The temperature in the geyser tube increases as we descend.