696 DYNAMICAL GEOLOGY. 



summer; and water is derived also from melting that takes place 

 over the surface of the glacier. Another source of water is stated 

 under d. 



(c.) The capability of sliding along its bed, but only portions at a 

 time. 



(d.) Pressure lowers the freezing point of water, so that with in- 

 crease of pressure, greater cold is required to keep ice solid ; hence, 

 wherever there is a strain occasioned by the obstacles to movement in 

 a glacier, the ice melts, relieving the resistance and facilitating motion. 

 (J. Thomson.) 



Rev. Henry Moseley, in a memoir on the cause of the descent of glaciers, treats of the 

 shearing force of ice, or the resistance to disruption which proceeds from the cohesion of 

 the mass; and he states, as the result of his trials with cylinders of ice having one end 

 fixed, that the amount of force required for shearing one square inch of ice is about 75 

 pounds. He arrives, in his calculations, at the conclusion that a force exceeding 34 

 times the weight of the glacier is demanded to shear the ice and cause the rate of de- 

 scent which Tyndall observed for the Mer de Glace at the Tacul. The results of his 

 trials varied with the time allowed, 120 pounds being found to be required for a quick 

 effect; and they did not recognize the variations in texture in the ice of a glacier. It 

 has been questioned whether the shearing-force would not be wholly lost in case the 

 time were given which an actual glacier takes. The deformations produced by even a 

 small amount of pressure (see a, above) in slow processes, may be all the yielding that 

 is needed; and this deformation is aided by the lowering of the freezing points by 

 pressure, as urged by Professor James Thomson. With regard to the motion, Canon 

 Moseley adopts the view that glaciers descend as a plate of lead descends a sloping sur- 

 face, through alternate changes of temperature, the movement from expansion by heat 

 being mainly downward because of gravity, and contraction working the same way. 

 To this theory the obvious objection holds, as has been observed, that glaciers do not 

 undergo the needed change of temperature. 



Professor Croll, in his Climate and Time (and in earlier memoirs) accepts of Moseley's 

 deduction as to the shearing force of ice, and brings forward a molecular theory to account 

 for the motion of glaciers. He says: " We find that the heat applied to one side of a 

 piece of ice will affect the thermal pile on the opposite side ; " and explains this, not 

 by radiation through the ice, but on the view that the heat applied passes from molecule 

 to molecule through the mass; the transmission of the heat-energy conveyed by A to 

 B melts B, but crystallizes A, and so it goes on through the ice. Professor Croll next 

 observes that a molecule in the liquid state takes up about one tenth less space than 

 in the solid, and hence, as it will obey gravity, it will move downward in the ice, and 

 consequently crystallize at a lower position than it before occupied ; and in crystalliz- 

 ing in its new position it will expand again. " All that the molecule requires is simply 

 room or space to advance in." "Each molecule of the glacier consequently descends 

 step by step as it melts and solidifies, and hence the glacier, considered as a mass, is in 

 a state of constant motion downward." Gravitation is the source of motion; the ex- 

 pansion of the crystallizing molecule aids it; and the shearing-force is lost by the mole- 

 cular melting. But it seems to be hardly probable that a glacier, hundreds of feet thick, 

 could be thus urged forward. Any crevasse or crack would intercept the molecular 

 transmission ; and the cause would hardly have a chance to act in a crevassed glacier 

 like the Mer de Glace. Professor Croll, however, explains the formation of crevasses 

 on the same principle. 



(7.) Crevasses. — Crevasses are made along the sides of a glacier, 

 especially when passing prominent angles in the valley, or over places 



