=The summer wave.=—The warm wave follows the analogy of the summer wave of ice-free land surfaces much less closely. This is because of the low melting temperature of ice as compared with other forms of solid earth-matter. On this account the summer wave is bi-fold. The one part travels downward by conduction, the other by the descent of water; the one has to do primarily with the temperature before the melting-point of ice is reached; the other, with the temperature after that point is reached; the first conforms measurably to the warm wave affecting other solid earth-matter, while the second is governed by special laws. After the surface portion of the ice is brought to the melting temperature, the additional heat which it receives melts the ice and is transformed from _sensible_ into _potential_ heat. Ice charged with water is _potentially_, but not _sensibly_, warmer than ice which has just reached the melting temperature. The warm wave of conduction dies out below like the cold wave. The warm wave descending by the flow of water stops where the freezing temperature of water is reached. In regions where the average temperature is below freezing, the water-wave does not descend so far as the wave of conduction, since the latter descends below the zone where the melting temperature is found. The foregoing considerations warrant the generalization that glaciers normally consist of two zones (1) an outer or upper zone of fluctuating temperature, and (2) an under zone of nearly constant temperature. The under zone obviously does not exist where the thickness of the ice is less than the thickness of the zone of fluctuating temperature. This may be the case in very thin glaciers in very cold regions, and in the thin ends and edges of all glaciers. =The temperature of the bottom.=—The internal heat of the earth is slowly conducted to the base of a glacier where it melts the ice at the estimated average rate of about one-fourth of an inch per year. The temperature of melting is a little below 32° Fahr. since pressure lowers the melting-point at the rate of .0133° Fahr. (.0075° C.) for one atmosphere of pressure. At the bottom of a mile of ice therefore the melting temperature is about 30.2° Fahr. (-1° C.) It is probable that in all thick glaciers the temperature of the bottom is constantly maintained at the melting-point. This may be indicated by the streams which issue from beneath glaciers during the winter, though this criterion is hardly decisive since the issuing waters may be derived partly or wholly from the rock beneath. In glaciers or in parts of glaciers so thin as to lie wholly within the zone of fluctuating temperature, the temperature of the bottom is obviously not constant.