IN CLOUDS AND RIVERS, ICE AND GLACIERS. 



FIG. 8. 



In other words, the strain upon no' is greater 

 than that upon m o' ; so that if one of them 

 were to break under the strain, it would be 

 the short one. 



280. These two lines represent the condi-, 

 tious of strain upon the two sides of the gla- 

 cier. The sides are held back, and the cen- 

 tre tries to move on, a strain being thus set 

 up between the centre and sides. But the 

 displacement of the point of maximum mo- 

 tion through the curvature of the valley 

 makes the strain upon the eastern ice greater 

 than that upon the western. The eastern 

 s..".c of the glacier is therefore more crevassed 

 than the western. 



281. Here indeed resides the difficulty of 

 getting along the eastern side of the Mer de 

 Glace : a difficulty which was one reason for 

 our crossing the glacier opposite to the Mon- 

 tanvert. There are two convex sweeps on 

 the eastern side to one on the western side, 

 hence on the whole the eastern side of the 

 Mer de Glace is most riven. 



g 43. MORAINE- RIDGES, GLACIER TABLES, 

 AND SAND CONES. 



282. When you and I first crossed the "Mer 

 de Glace from Trelapoite to the Couvercle, 

 we I'ound that the stripes of rocks and rub- 

 bish which constituted the medial moraines 

 were ridges raised above the general level of 

 the glacier to a height at some places of 

 twenty or thirty feet. Oil examining these 

 ridges we found the rubbish to be superficial, 

 and that it rested upon a great spine of ice 

 which ran along the back of the glacier. By 

 what means has this ridge of ice been raised V 



283. Most boys have read the story of Dr. 

 Franklin's placing bits of cloth of various 

 colors upon snow on a sunny day. The bits 

 of cloth sank in the snow, the dark ones 

 jnost. 



284. Consider this experiment. The sun's 

 rays first of all fail upon the upper surface 

 of the cloth and warm it. The neat is then 

 conducted through the cloth to the under 

 surface, and the under surface passes it oil to 

 the snow, which is finally liquefied by the 

 heat. It is quite manifest that the quantity 

 of snow melted will altogether depend upon 

 the amount of heat sent from tiie upper to 

 the under surface of the cloth. 



285. Now cloth is what is called a bad 

 conductor. It does not permit heat to travel 

 freely through it. B .i where it has merely 

 to pass through the thickness of a single bit 

 of cloth, a jjood quantity of the heat y;eta 



through. But if you double or treble or 

 quintuple the thickness of the cloth ; or, 

 what is easier, if you put several pieces one 

 upon the other, you come at length to a 

 point where no sensible amount of heat could 

 get through from the upper to the under sur- 

 face. 



286. What must occur if such a thick 

 piece, or such a series of pieces of cloth, 

 were placed upon snow on which a strong 

 sun is falling? The snow round the cloth is 

 melted, but that underneath the cloth is pro- 

 tected. If the action continue long enough 

 the inevitable result will be that the level of 

 the snow all round the cloth will sink, and 

 the cloth will be left behind perched upon 

 an eminence of snow. 



287. If you understand this, you have al- 

 ready mastered the cause of the moraine- 

 ridges. They are not produced by any 

 swelling of the ice upward. But the ice 

 underneath the rocks and rubbish being pro- 

 tected from the sun, the glacier right and 

 left melts away and leaves a ridge behind. 



288. Various other appearances upon the 

 glacier are accounted for in the same way. 

 Here upon the Mer de Glace we have flat 

 slabs of rock sometimes lifted up on pillars 

 of ice. These are the so-called Glacier Ta- 

 bks. They are produced, not by the growth 

 of a stalk of ice out of the glacier, but by 

 the melting of the glacier all round the icu 

 protected by the stone. Here is a sketch of 

 one of the Tables of the Mer de Glace. 



289. Notice moreover that a glacier table 

 is hardly ever set square upon its pillar. It 

 generally leans to one side, and repeated ob- 

 servation teaches you that it so leans as to 

 enable you always to draw the north and 

 south line upon the glacier. For the sun be- 

 ing south of the zenith at noon pours its rays 

 against the southern end of the table, while 

 the northern end remains in shadow. The 

 southern end, therefore, being most warmed 

 does not protect the ice underneath H so ef- 

 fectually as the northern end. The table be- 

 comes inclined, and ends by sliding bodily 

 olf its pedestal. 



290. In the figure opposite we have what 

 may be called an ideal table. The oblique 

 lines represent the direction of the sunbeams, 

 and the consequent tilting of the table here 

 shown resembles that observed upon t^ 

 glaciers. 



291. A pebble will not rise thus : like 

 Franklin's single bit of cloth, a dark-colored 

 pebble sinks in the ice. A spot of black 

 mould will not rest upon the surface, but 

 will sink ; and various parts of the Glacier du 

 Geant are honeycombed by the sinking of 

 such spots of dirt into the ice. 



292. "But when the dirt is of a thickness 

 sufficient to protect the ice the case is differ- 

 ent. Sand is often washed away by a stream 

 from the mountains, or from the moraines, 

 and strewn over certain spaces of the glacier. 

 A most curious action follows . the sanded 

 surface rises, the part on which the sand lies 

 thickest rising highest. Little peaks and 

 eminences jut forth, and when the distribi*- 



