Makch 1, 1895.] 



KNOWLEDGE. 



67 



given up again as soon as the water reassumed a crystalline 

 form. One other question may very naturally occur to the 

 reader, namely, if heat is required to melt ice, how is it 

 that water and ice can exist separately at the same 

 temperature '? In the first place, water at freezing point 

 is in ail incipient state of crystallization ; and secondly, 

 from the commencement to the end of solidification the 

 temperature of a liquid remains constant. 



Let us now attempt to apply the foregoing principles of 

 the expansion of ice on a small scale to the movement of 

 glaciers, and other great ice-sheets. 



The sun, in expending its rays on the surface of the 

 glacier, does not raise the temperature of the ice of which 

 it is composed ; it, however, imparts energy, which, with 

 gravitation, causes the glacier to move along that path 

 where, as a whole, it finds least resistance to its motion. 



The method may be thus described: — The heat of 

 the sun melts the ice on the surface of the glacier, 

 and it contracts in bulk on becoming liquid, and flows 

 downwards by the aid of gravitation into the interstices 

 between the ice-crystals below. Here the water is no 

 longer influenced by the sun's rays, and again becomes 

 crystalline, but the crannies and corners into which it 

 has found its way are not suitable in shape to contain 

 it in the form of crystals, and therefore in parcmg with 

 its heat it employs that irresistible force due to crystalli- 

 zation to make the cavities larger. In other words, it 

 pushes away the molecules surrounding it down the path 

 of least resistance. But we must not forget that the 

 molecules of water on becoming re-crystallized part with 

 latent heat. This heat is taken up by adjacent molecules 

 of ice, which, in their turn, become water, flow downwards 

 and exert pressure in the process of re-crystallization. 

 Thus, little by little, and from molecule to molecule, the 

 heat derived from the sun is transmitted through the 

 length and breadth of the glacier. The action is not at 

 one place alone, but permeates the whole mass. We 

 must look upon the glacier as an agglomeration of moving 

 molecules, which, having utilized gravitation to enable 

 them to flow downwards when in a liquid state, follow up 

 that movement by an overpowering pressure produced 

 during their process of re-crystallizatiou. 



But liow is it, if a glacier is constantly receiving heat 

 on its surface which passes through its entire thickness, 

 that it does not in time all melt '? Like other rivers, 

 glaciers are constantly being fed with fresh material 

 from their sources and by tributaries, and this supply 

 neutralizes the waste which actually does take place, and 

 which is represented by the terminal streams of water — 

 the origin of some of the greatest rivers in the world. A 

 glacier receives heat and energy from other sources than 

 the direct rays of the sun on its surface. Radiation from 

 the earth, and tepid water entering the larger fissures of 

 the ice, convey heat into the interior which, in its action, 

 produces exactly similar efl'eets to those caused by the 

 direct rays of the sun. 



If this molecular theory of the movement of ice is the 

 true one, there is no difficulty in accounting for the 

 peculiar manner in which glaciers adapt their shape to the 

 inequalities of the ground over which they flow. It is true 

 that crevasses may be formed, o-wing to a sudden alteration 

 of slope producing a transverse strain across an ice stream, 

 and thus breaking it in two or forming deep cracks (to be 

 healed, eventually, by the process of regelation) before the 

 molecular movement can adapt the glacier to the form it is 

 required to take ; but if time is allowed, ice has shown 

 itself to possess wonderful qualities in adapting itself to 

 the nature of the valleys in which it moves. 



Perhaps the most striking examples of this kind are 



furnished, not by existing glaciers, but by those of the last 

 glacial period, when on Northern Europe the land ice 

 extended much farther south than it does at present, and 

 was of enormous weight and thickness. During that 

 period, rock basins, which now form large lakes, were filled 

 with ice, and in some instances it has been shown that 

 they were carved out of hard rock by glacial action. 



The directions in which the ice moved during this cold 

 period in Northern Europe is ascertained from the marks 

 it caused to be made on the hard rocks over which it 

 passed. The striae are a certain indication of the trend of 

 the ice-flow, and prove that the ice not only travelled 

 down the slope into the rock basin, but that it also ruse up 

 the slope at the other end, although the surface of the 

 glacier might be several hundred feet above. 



This fact demonstrates more conclusively than any other 

 example that could be given, the complex molecular move- 

 ment in every part of a glacier. If the ice had moved 

 by pressure from behind, and by gravitation, as it would 

 affect a mass of plastic clay, it would have first filled 

 the lake basin, and then continued to flow over the top. 

 But this was not the case. It moved on the top, it moved 

 in the middle, and it moved at the bottom ; each molecule 

 in the entire mass was constantly taking in energy from 

 heat and acting alone, and giving out energy and acting 

 in conjunction with its neighbours in converting that 

 energy into motion. 



With regard to the striated surface of some of 

 the harder rocks over which a glacier has in bygone ages 

 flowed, it might at first be thought that these markings 

 pointed more to a sliding than to molecular motioQ. 

 It must, however, be remembered that the striae are 

 produced, not by the ice itself, but by graving tools 

 in the form of stones, which it carries with it in its 

 general motion. These stones, loosened from the ground, 

 become imbedded in the ice, cutting and grooving the 

 solid rock surfaces beneath, they themselves being ground 

 down in the process. 



There is another circumstance which tends to prove 

 that glaciers in their interior are not inert masses of ice. 

 This is the habit they have of disgorging stones and other 

 foreign matter on their surface. An example of this 

 tendency is that a knapsack, having been dropped into a 

 crevasse, found its way to the surface of a confluent 

 glacier ten years later, and more than three-quarters of a 

 mile from the place where it had been lost. 



There is, it appears, no means of arresting the move- 

 ment of glaciers except by cutting off entirely their supply 

 of heat, and this for a sufficient time to allow them to get 

 rid of that which they have already absorbed. 



Perhaps the most interesting direction in which these 

 considerations lead us is towards the study of the action of 

 glaciers and ice-sheets during the last and former glacial 

 periods. Evidence of this action is to be found on every 

 hand, and it is often strange to think as we stand in the 

 warm sunshine in a pleasant wooded valley, decked with 

 every variety of vegetation, that about the time when man 

 apparently first appeared on the earth the place lay 

 himdreds of feet below a great sea of ice which over- 

 spread Northern Europe. 



Enough has been said to draw the attention of the 

 reader to the means employed by Nature for carrying out 

 the work which she desires to perform. She employs 

 no labour nor constructs elaborate machines to do her 

 will, but, thi-ough the reciprocal relationship of the physical 

 forces, causes great changes to take place in the universe, 

 which, simple as they may seem to be in the abstract, like 

 the movement of glaciers, are, though complex in the 

 extreme, superbly accurate and efficient in every detail. 



