The Theory of Glacier motion. 129. 



various degrees of plasticity within narrower limits, in 

 the same manner that wax, for example, softens before it 

 melts. M. Person deduces this from the examination of 

 the heat requisite to liquefy ice at different temperatures. 

 The following sentences contain his conclusions in his own 

 words : " II parait d'apres mes experiences que le ramollis- 

 sement qui precede la fusion, est circonscrit dans une 

 intervalle d'environ 2 degres. La glace est done un des 

 corps dont la fusion est la plus nette ; mais cependant 

 le passage de I'etat solide a I'etat liquide s'y fait encore par 

 degres, et non par un saut brusque." Forbes adds, that 

 from his own experiments and those of Agassiz, it is 

 clear that the normal temperature of the great mass of 

 a glacier is between 28° and 32° Fahrenheit, that the coldest 

 nights only affect the temperature of its superficial layers, 

 that the lower parts which are habitually saturated with 

 water in summer are seldom reduced below the freezing 

 point, even by the prolonged cold of winter, since it is. 

 then covered with snow which has the property of prevent- 

 ing any profound congelation in common earth ; and as ice 

 is probably a better conductor of heat than the ground,, 

 it is incredible that a thickness of many hundred feet 

 of ice, saturated with fluid water, should be reduced 

 much below the freezing point, or should even be frozen 

 throughout. The fact that water continually flows from, 

 under glaciers in winter shews that they are not so frozen. 

 While the fact that, even in February, the source of the 

 Arveiron becomes whitish and dirty as in summer before a 

 change of weather, proves that in the middle of winter a 

 temporary rise of temperature over the higher glacier 

 regions, not only produces a thaw there, but finds the usual 

 channels still open for transmitting the accumulated snow- 

 water. It thus appears quite certain that ice, under the 

 circumstances in which we find it in the great bulk of 

 glaciers, is in a state more or less softened cwqxx in winter ; and 



