Edinburgh New Philosophical Journal (1844 Edinburgh New Philosophical Journal edinburghnewphil38edin Year: 1844 - 1845 Professor Forbes's Ninth Letter on Glaciers. 335 slaty cleavage of the ice, in the way that I have explained in my Seventh Letter. In a glacier then, whose slope is nearly constant and small, I should expect a condensation of the ice longitudinally, and a swelling of the surface depending upon the motion of the plastic ice in the direction of least resist- ance. Now this is exactly what we have in the results of measurement. If the annexed figure represent the plan wr^ijr
Edinburgh New Philosophical Journal (1844 Edinburgh New Philosophical Journal edinburghnewphil38edin Year: 1844 - 1845 Professor Forbes's Ninth Letter on Glaciers. 335 slaty cleavage of the ice, in the way that I have explained in my Seventh Letter. In a glacier then, whose slope is nearly constant and small, I should expect a condensation of the ice longitudinally, and a swelling of the surface depending upon the motion of the plastic ice in the direction of least resist- ance. Now this is exactly what we have in the results of measurement. If the annexed figure represent the plan wr^ijr ^imMmjri of the glacier, and the ice be divided into imaginary com- partments by vertical sections; since, whilst AB moves 177 feet, CD moves but 141 feet, there is a condensation of the mass of ice A B C D, from back to front, of no less than 36 feet in that time, and so for the successive slices E F, &c. How, then, is this shrinking to be accounted for \ Not by the mere internal melting, for that would pro- duce merely a lowering of the surface, and a subsidence of the level of the ice ; such as I have shewn* actually takes place in other glaciers whose sections move with increasino> velocity on the whole. There is only a vis a tergo which can approximate the sections together, and, as we read in the Comptes Rendus, squeeze the moraine longitudinally, giv- ing it a greater breadth,t and condense the entire body of the ice so as to make it more compact in texture. If we take a vertical section instead of a plan (see next page), the slice abed must be condensed into the hio-her and shorter solid c d ef, and so of the rest, and the surface * Travels, p. 153. t Les ' Moraines me'dianes s'e'largissent dans la meme proportion que le mouvement se ralentit.' Comptes Rendus, 9th Dec. 1844, p. 1301. X lb. p. 1306, line 29.
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