279. Let A B and C D, in the diagram opposite, represent the two curved sides of the Mer de Glace at the Montanvert, and let _m n_ be a straight line across the glacier. Let _o_ be the point of maximum motion. The mechanical state of the two sides of the glacier may be thus made plain. Supposing the line _m n_ to be a straight elastic string with its ends fixed; let it be grasped firmly at the point o by the finger and thumb, and drawn to _o'_, keeping the distance between _o'_ and the side C D constant. Here the length, _n o_ of the string would have stretched to _n o'_, and the length _m o_ to _m o'_ and you see plainly that the stretching of the short line, in comparison with its length, is greater than that of the long line in comparison with its length. In other words, the strain upon _n o'_ 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. [Illustration: _Montanvert_] 280. These two lines represent the conditions of strain upon the two sides of the glacier. The sides are held back, and the centre tries to move on, a strain being thus set up between the centre and sides. But the displacement of the point of maximum motion through the curvature of the valley makes the strain upon the eastern ice greater than that upon the western. The eastern side 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 Montanvert. 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. § 43. _Moraine-ridges, Glacier Tables, and Sand-Cones._ 282. When you and I first crossed the Mer de Glace from Trélaporte to the Couvercle, we found that the stripes of rocks and rubbish 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. On 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? 283. Most boys have read the story of Dr. Franklin's placing bits of cloth of various colours upon snow on a sunny day. The bits of cloth sank in the snow, the dark ones most. 284. Consider this experiment. The sun's rays first of all fall upon the upper surface of the cloth and warm it. The heat is then conducted through the cloth to the under surface, and the under surface passes it on 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 the upper to the under surface of the cloth.