306 GLACIAL GKAVELS OF MAINE. 



amount of mechanical work was inevitably accompanied by a considerable 

 development of lieat from friction. Its quantity would depend on many 

 variables, sucli as the coefficient of friction of the ice against different kinds 

 of rock, the pressure and rate of motion of the ice, the amount of englacial 

 matter, etc. It is well known that beneath landslides and avalanches con- 

 siderable frictional heat is developed. Whether the heat generated by the 

 slower motion of the snow and ice will cause basal melting depends on the 

 basal temperature of the mass. Where available for melting, heat from this 

 cause might considerably augment the basal waters, but the quantity is 

 unknown. 



4. Basal melting due to heat transmitted from above through the ice. 



Croll's theory of glacial motion seems to involve the hypothesis that 

 heat can be transmitted from a particle of water to a particle of ice without 

 a difference of temperature to act like the electromotive force to drive it. 

 Without involving ourselves in dynamical questions, we can for the time con- 

 sider the ice as static, and assume that the passage of molecular heat in it is 

 from particle to particle by the process of conduction from where there is a 

 higher to a lower temperature. It follows, since all the lower portions of 

 glaciers have the temperature of 32°, that the heat contained in the ice can 

 not, unless pressure changes the melting point, pass out of one part of the 

 ice to produce melting of another part of the same body of ice. Omitting 

 from the present discussion the questions involved in the varying melting 

 point of ice under varying pressures, we are justified in the conclusion that 

 molecular heat from the surface Avill be conducted downward until the tem- 

 perature of all the mass is at 32°, and then no more can pass, for the ten- 

 sion, to use the electrical term, is then equally high in every part. But in 

 the form of ether vibrations energy can penetrate the ice irrespective of 

 temperature. The rougher and more granular condition of the ice near the 

 surface indicates that most of the radiant heat is absorbed soon after passing 

 into the ice — i. e., is converted into molecular heat and causes melting at a 

 multitude of places. The reflections from the surfaces of these cavities 

 containing water causes the opaque and granular appearance of surface ice. 

 But it is well known that the words "transparent" and "opaque" are rela- 

 tive terms, referring only to visual rays, not to all the waves of ether energy. 

 It seems probable that the rays capable of producing photographic effects 

 on silver salts, and all the rays visual to the eye, are absorbed by water 



